NullPointerException when annotating com.google.cloud:spring-cloud-gcp-dependencies

While checking takari-local-repository, I found NullPointerException ([memo](https://gist.github.com/suztomo/ca194d3cdd00072e2c2d57c17a640056)). Even with the latest 1.5.4 Linkage Checker enforcer rule, the problem (https://github.com/GoogleCloudPlatform/cloud-opensource-java/issues/1609) recurs

```
suztomo@suztomo:~/spring-cloud-gcp$ mvn --activate-profiles linkage-check\
--batch-mode --show-version\
--threads 1.5C --define skipTests=true\
--define maven.javadoc.skip=true --errors install

...

suztomo@suztomo:~/spring-cloud-gcp$ git remote -v
origin https://github.com/GoogleCloudPlatform/spring-cloud-gcp.git (fetch)
origin https://github.com/GoogleCloudPlatform/spring-cloud-gcp.git (push)
suztomo@suztomo:~/spring-cloud-gcp$ git diff
diff --git a/spring-cloud-gcp-dependencies/pom.xml b/spring-cloud-gcp-dependencies/pom.xml
index b0752ca0..2050a6d8 100644
--- a/spring-cloud-gcp-dependencies/pom.xml
+++ b/spring-cloud-gcp-dependencies/pom.xml
@@ -370,7 +370,7 @@

com.google.cloud.tools
linkage-checker-enforcer-rules
- 1.5.3
+ 1.5.4

[INFO] Spring Cloud GCP Dependencies ...................... FAILURE [03:58 min]
[INFO] Spring Cloud GCP Documentation ..................... SUCCESS [ 5.141 s]
[INFO] Spring Cloud GCP Code Samples ...................... SUCCESS [ 6.414 s]
[INFO] Spring Cloud GCP Code Sample - Runtime Configuration SUCCESS [ 0.863 s]
[INFO] Spring Cloud GCP Code Sample - Trace ............... SUCCESS [ 1.134 s]
[INFO] Spring Cloud GCP Code Sample - Logging ............. SUCCESS [ 5.685 s]
[INFO] Spring Cloud GCP Code Sample - MySQL ............... SUCCESS [ 0.901 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Pub/Sub Channel Adapters SUCCESS [ 1.726 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Pub/Sub Sender SUCCESS [ 6.948 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Pub/Sub Receiver SUCCESS [ 3.521 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Pub/Sub Polling Receiver SUCCESS [ 0.939 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Pub/Sub Integration Test SUCCESS [ 1.256 s]
[INFO] Spring Cloud GCP Code Sample - Storage ............. SUCCESS [ 4.935 s]
[INFO] Spring Cloud GCP Code Sample - Spanner ............. SUCCESS [ 6.570 s]
[INFO] Spring Cloud GCP Code Sample - Datastore ........... SUCCESS [ 1.401 s]
[INFO] Spring Cloud GCP Code Sample - Datastore Bookshelf . SUCCESS [ 1.170 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub ............. SUCCESS [ 5.817 s]
[INFO] Spring Cloud GCP Code Sample - Spring Data JPA ..... SUCCESS [ 5.201 s]
[INFO] Spring Cloud GCP Code Sample - Vision API .......... SUCCESS [ 1.230 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Storage SUCCESS [ 5.742 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Stream ...... SUCCESS [ 2.143 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Bus Configuration Management SUCCESS [ 1.725 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Bus (Client) SUCCESS [ 1.453 s]
[INFO] Spring Cloud GCP Code Sample - Spring Cloud Config with Pub/Sub Bus (Local Configuration) SUCCESS [ 1.453 s]
[INFO] Spring Cloud GCP Code Sample - Spring Cloud Config with Pub/Sub Bus (GitHub Configuration) SUCCESS [ 1.452 s]
[INFO] Spring Cloud GCP Code Sample - Spring Cloud Config with Pub/Sub Bus Integration Test SUCCESS [ 1.620 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Stream Binder Polling SUCCESS [ 2.143 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Reactive .... SUCCESS [ 6.643 s]
[INFO] Spring Cloud GCP Code Sample - Spring Integration Pub/Sub JSON Payloads SUCCESS [ 6.803 s]
[INFO] Spring Cloud GCP Code Sample - IAP ................. SUCCESS [ 0.873 s]
[INFO] Spring Cloud GCP Code Sample - Postgres ............ SUCCESS [ 6.156 s]
[INFO] Spring Cloud GCP Code Sample - Vision OCR .......... SUCCESS [ 5.332 s]
[INFO] Spring Cloud GCP Code Sample - Firestore ........... SUCCESS [ 0.968 s]
[INFO] Spring Cloud GCP Code Sample - Multi-Spring-Data-Module SUCCESS [ 1.103 s]
[INFO] Spring Cloud GCP Code Sample - Spring Data Firestore SUCCESS [ 6.507 s]
[INFO] Spring Cloud GCP Code Sample - BigQuery ............ SUCCESS [ 6.511 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Stream Binder (Functional style) SUCCESS [ 1.713 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Stream Binder Source SUCCESS [ 2.191 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Stream Binder Sink SUCCESS [ 2.191 s]
[INFO] Spring Cloud GCP Code Sample - Pub/Sub Stream Binder Integration Test SUCCESS [ 1.465 s]
[INFO] Spring Cloud GCP Code Sample - Firebase Security ... SUCCESS [ 5.542 s]
[INFO] Spring Cloud GCP Code Sample - Secret Manager ...... SUCCESS [ 1.358 s]
[INFO] Spring Cloud GCP Code Samples - Kotlin ............. SUCCESS [ 1.732 s]
[INFO] Spring Cloud GCP Code Sample - Kotlin App Sample ... SUCCESS [ 14.329 s]
[INFO] Spring Cloud GCP Code Sample - Metrics ............. SUCCESS [ 0.934 s]
[INFO] Spring Cloud GCP Code Sample - KMS ................. SUCCESS [ 1.068 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 04:00 min (Wall Clock)
[INFO] Finished at: 2020-12-29T22:06:59Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce (enforce-linkage-checker) on project spring-cloud-gcp-dependencies: Execution enforce-linkage-checker of goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce failed. NullPointerException -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce (enforce-linkage-checker) on project spring-cloud-gcp-dependencies: Execution enforce-linkage-checker of goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce failed.
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:215)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call (MultiThreadedBuilder.java:190)
at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call (MultiThreadedBuilder.java:186)
at java.util.concurrent.FutureTask.run (FutureTask.java:266)
at java.util.concurrent.Executors$RunnableAdapter.call (Executors.java:511)
at java.util.concurrent.FutureTask.run (FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:624)
at java.lang.Thread.run (Thread.java:748)
Caused by: org.apache.maven.plugin.PluginExecutionException: Execution enforce-linkage-checker of goal org.apache.maven.plugins:maven-enforcer-plugin:3.0.0-M3:enforce failed.
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:148)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call (MultiThreadedBuilder.java:190)
at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call (MultiThreadedBuilder.java:186)
at java.util.concurrent.FutureTask.run (FutureTask.java:266)
at java.util.concurrent.Executors$RunnableAdapter.call (Executors.java:511)
at java.util.concurrent.FutureTask.run (FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:624)
at java.lang.Thread.run (Thread.java:748)
Caused by: java.lang.NullPointerException
at com.google.cloud.tools.opensource.dependencies.DependencyGraph.levelOrder (DependencyGraph.java:259)
at com.google.cloud.tools.opensource.dependencies.DependencyGraph.from (DependencyGraph.java:244)
at com.google.cloud.tools.opensource.dependencies.DependencyGraphBuilder.buildDependencyGraph (DependencyGraphBuilder.java:209)
at com.google.cloud.tools.opensource.dependencies.DependencyGraphBuilder.buildMavenDependencyGraph (DependencyGraphBuilder.java:198)
at com.google.cloud.tools.opensource.classpath.ClassPathBuilder.resolveWithMaven (ClassPathBuilder.java:80)
at com.google.cloud.tools.opensource.classpath.LinkageProblemCauseAnnotator.annotate (LinkageProblemCauseAnnotator.java:59)
at com.google.cloud.tools.dependencies.enforcer.LinkageCheckerRule.execute (LinkageCheckerRule.java:223)
at org.apache.maven.plugins.enforcer.EnforceMojo.execute (EnforceMojo.java:205)
...
```

As per the stacktrace Linkage Checker was trying to annotate linkage errors (`LinkageProblemCauseAnnotator.annotate`).

Note that this happens without using takari local repository extension.