NullPointerException caused by empty collection attribute in incremental flushing

The following test case gives a NullPointerException due to bugs in the incremental attribute flushing:

A ::= B* C*;
B ::= <NAME>;
C ::= <NAME> <TO:Integer>;

aspect Test {
  inh A C.a();
  eq A.getChild().a() = this;

  coll HashSet<C> B.cs() root A;

  C contributes this
      to B.cs()
      for a().getB(getTO());
}

public class Test {
  public static void main(String[] args) {
    int N = 6;
    A a = new A();
    for (int i = 0; i < N; ++i) {
      a.addB(new B("b"+i));
    }

    a.addC(new C("c1", 1));
    a.addC(new C("c2", 2));
    a.addC(new C("c3", 3));
    a.addC(new C("c4", 5));

    checkCollections(a, "before", 0, 1, 1, 1, 0, 1);

    // Transform:
    a.getBList().removeChild(2);

    checkCollections(a, "after", 0, 1, 1, 0, 1);
  }

  static void checkCollections(A a, String phase, int... sizes) {
    for (int i = 0; i < sizes.length; ++i) {
      testEqual(phase + "@" + a.getB(i).getNAME(), sizes[i], a.getB(i).cs().size());
    }
  }

  static void testEqual(String msg, int a, int b) {
    if (a != b) {
      System.out.format("Error %s : %d != %d%n", msg, a, b);
    }
  }
}

Error output:

Exception in thread "main" java.lang.NullPointerException
    at ast.B.reactToDependencyChange(B.java:196)
    at ast.ASTNode$DepGraphNode.dependencyChanged(ASTNode$DepGraphNode.java:164)
    at ast.ASTNode$DepGraphNode.notifyDependencies(ASTNode$DepGraphNode.java:150)
    at ast.ASTNode.removeChild(ASTNode.java:312)
    at ast.List.removeChild(List.java:72)
    at Test.main(Test.java:26)

The dependency tracking for collection attributes is incorrect right now and should be fixed. It is easy to fix the NPE but that alone will not fix the incremental update of collection attributes.

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