Pipeline fails to 'Sign' for Android in Mobile CI/CD

## Expected Behavior
The pipeline completes successfully

## Actual Behavior
Pipeline run fails in the `signAndroidApks` step in the 'Sign' stage, with the following output:

```

BUILD SUCCESSFUL in 34s
45 actionable tasks: 45 executed
Built the following apk(s):
/tmp/workspace/myproject/myproject-android-release/platforms/android/app/build/outputs/apk/release/app-release-unsigned.apk

[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Sign)
[Pipeline] signAndroidApks
[myproject-android-release] $ echo "resolving effective environment"
[Pipeline] }
[Pipeline] // stage
[Pipeline] }
[Pipeline] // node
[Pipeline] End of Pipeline
java.lang.NullPointerException
at com.cloudbees.plugins.credentials.matchers.IdMatcher.(IdMatcher.java:56)
at com.cloudbees.plugins.credentials.CredentialsMatchers.withId(CredentialsMatchers.java:132)
at org.jenkinsci.plugins.androidsigning.SignApksBuilder.getKeystore(SignApksBuilder.java:383)
at org.jenkinsci.plugins.androidsigning.SignApksBuilder.perform(SignApksBuilder.java:286)
at org.jenkinsci.plugins.androidsigning.SignApksStep$SignApksStepExecution.run(SignApksStep.java:174)
at org.jenkinsci.plugins.androidsigning.SignApksStep$SignApksStepExecution.run(SignApksStep.java:126)
at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1$1.call(AbstractSynchronousNonBlockingStepExecution.java:47)
at hudson.security.ACL.impersonate(ACL.java:290)
at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1.run(AbstractSynchronousNonBlockingStepExecution.java:44)
at java.util.concurrent.Executors$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)
Finished: FAILURE
```

## Steps to Reproduce the Problem

1. Create a build for the master branch of this repo in OpenShift, for the Mobile CI/CD Service
1. Choose Android for the Platform, and 'Release' as the Build Type (don't tick the "Upload external credentials (Use this option if your Keystore is external to your source code) " checkbox
1. Start a build and wait for it to fail at the Sign stage

If it's necessary to check that 'external credentials' box and give more input there, then maybe that shouldn't be optional (I have no idea if that's necessary or not). In either case, it might be better if it failed with something more useful than a NullPointerException.