fix: NPE in JDTCommentBuilder

[Egor18]

Hello.
I was trying to migrate from spoon 7.0 to 7.2, and encountered some mode build failures due to the recent changes. (We have a large test base with ~80 open-source projects).

Spoon 7.2 failed to build a model with NPE for the following code (it's from sakai open-source project):

if (itemGrading.getAttemptsRemaining() == null){
    // this is not possible unless the question is submitted without the
    // attempt set correctly
    if ((item.getTriesAllowed()).intValue() >= 9999)
      attemptsRemaining = 9999;  
} else{
  if ((item.getTriesAllowed()).intValue() >= 9999 )
    attemptsRemaining = 9999;
  else if (itemGrading.getAttemptsRemaining().intValue() > 0);
  // We're now getting the applet to tell us how many attempts remain
  // attemptsRemaining = itemGrading.getAttemptsRemaining().intValue() - 1;
  else
    throw new Exception("This page must have been reached by mistake. Our record shows that no more attempt for this question is allowed.");
}

Stacktrace:

[INFO] java.lang.NullPointerException
[INFO] 	at spoon.support.compiler.jdt.JDTCommentBuilder$1.visitCtIf(JDTCommentBuilder.java:376)
[INFO] 	at spoon.support.reflect.code.CtIfImpl.accept(CtIfImpl.java:45)
[INFO] 	at spoon.reflect.visitor.CtInheritanceScanner.scan(CtInheritanceScanner.java:184)
[INFO] 	at spoon.support.compiler.jdt.JDTCommentBuilder$1.scan(JDTCommentBuilder.java:233)
[INFO] 	at spoon.support.compiler.jdt.JDTCommentBuilder.insertCommentInAST(JDTCommentBuilder.java:433)
[INFO] 	at spoon.support.compiler.jdt.JDTCommentBuilder.buildComment(JDTCommentBuilder.java:147)
[INFO] 	at spoon.support.compiler.jdt.JDTCommentBuilder.build(JDTCommentBuilder.java:111)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.lambda$buildModel$2(JDTBasedSpoonCompiler.java:443)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.forEachCompilationUnit(JDTBasedSpoonCompiler.java:457)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.buildModel(JDTBasedSpoonCompiler.java:442)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.buildUnitsAndModel(JDTBasedSpoonCompiler.java:375)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.buildSources(JDTBasedSpoonCompiler.java:340)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.build(JDTBasedSpoonCompiler.java:123)
[INFO] 	at spoon.support.compiler.jdt.JDTBasedSpoonCompiler.build(JDTBasedSpoonCompiler.java:106)
[INFO] 	at spoon.Launcher.buildModel(Launcher.java:772)
[INFO] 	at com.pvsstudio.projects.Module.build(Module.java:283)
[INFO] 	at com.pvsstudio.runner.ModuleWorker.run(ModuleWorker.java:41)
[INFO] 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[INFO] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[INFO] 	at java.lang.Thread.run(Thread.java:748)

In general spoon 7.2 fails to build model for the following code:

if (false)
    System.out.println("hello");
else if (false);
    // Do nothing...
else
    System.out.println("world");

I think this issue is related to that commit: 7f29a81

So in this PR I provide failing test case and a possible solution (we need to check if then part is null, even if the else part is not null).

[pvojtechovsky]

Thank you Egor!

Could you please simplify the already complex and long conditions. E.g. by creating variables for thenExpression?

I think this issue is related to that commit: 7f29a81

no, it is related to #2733

[pvojtechovsky]

Thank you!