Confusing null reference exception when run in Maven without referenced base tree

[olafur-skulason]

When building an Overture code generation project with AstCreator, using maven.

AstCreator prints out "Extension base artifact found - exstracting base tree definition files" indicating that the extension base artifact has been found and then if no such artifact is specified a null pointer exception is thrown.

The problem occurs in: maven/src/main/java/org/overture/tools/maven/astcreator/GenerateTree.java in line 80.

To cause the issue a maven pom.xml which defines the plugin in the build. As in the Overture/vdm2c project, but not including an artifact dependeny providing the artifact indicated in extendedAstArtifactId.

To solve the issue I would propose adding before line 79 of maven/src/main/java/org/overture/tools/maven/astcreator/GenerateTree.java with along the lines of:

if(baseArtifact == null)
{
getLog().error("Base artifact not found. Are you missing an artifact dependency?");
return;
}