[MNG-6004] Emit better error message for simple XML typos

**[Archie Cobbs](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=archie172)** opened **[MNG-6004](https://issues.apache.org/jira/browse/MNG-6004?redirect=false)** and commented

When porting a project from ant to maven, I accidentally said this:

```

```

instead of this:

```

http://www.slf4j.org/api/

```

Easy enough mistake to make. However, the error message was utterly mystifying:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:2.9.1:jar (attach-javadocs) on project msrp4j: Execution attach-javadocs of goal org.apache.maven.plugins:maven-javadoc-plugin:2.9.1:jar failed. NullPointerException -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:2.9.1:jar (attach-javadocs) on project msrp4j: Execution attach-javadocs of goal org.apache.maven.plugins:maven-javadoc-plugin:2.9.1:jar failed.
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:224)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:120)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:355)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:155)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:584)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:216)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:160)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
Caused by: org.apache.maven.plugin.PluginExecutionException: Execution attach-javadocs of goal org.apache.maven.plugins:maven-javadoc-plugin:2.9.1:jar failed.
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:143)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
... 19 more
Caused by: java.lang.NullPointerException
at org.codehaus.plexus.util.xml.pull.MXSerializer.writeElementContent(MXSerializer.java:921)
at org.codehaus.plexus.util.xml.pull.MXSerializer.text(MXSerializer.java:785)
at org.apache.maven.plugin.javadoc.options.io.xpp3.JavadocOptionsXpp3Writer.writeJavadocOptions(JavadocOptionsXpp3Writer.java:268)
at org.apache.maven.plugin.javadoc.options.io.xpp3.JavadocOptionsXpp3Writer.write(JavadocOptionsXpp3Writer.java:70)
at org.apache.maven.plugin.javadoc.AbstractJavadocMojo.buildJavadocOptions(AbstractJavadocMojo.java:5867)
at org.apache.maven.plugin.javadoc.AbstractJavadocMojo.executeReport(AbstractJavadocMojo.java:1857)
at org.apache.maven.plugin.javadoc.JavadocJar.execute(JavadocJar.java:181)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:132)
... 20 more
```

Suggestion: include code that performs basic structural checks so as to make it impossible for a cryptic `NullPointerException` to occur simply due to incorrect POM input.

---

**Affects:** 3.3.9

1 votes, 4 watchers