[ggj][ast][engx] fix: validate non-null elements for AnonymousClassExpr

src/main/java/com/google/api/generator/engine/ast/AnonymousClassExpr.java

@@ -58,18 +58,26 @@ public Builder setMethods(MethodDefinition... methods) {
 
     public AnonymousClassExpr build() {
       AnonymousClassExpr anonymousClassExpr = autoBuild();
+
       // 1. The anonymous class expression should be reference types.
       Preconditions.checkState(
           TypeNode.isReferenceType(anonymousClassExpr.type()),
           "Anonymous class expression must be reference types.");
+
+      // 2. Check that there are no null methods or statements.
+      String contextInfo =
+          String.format("anonymous class of type %s", anonymousClassExpr.type().reference().name());
+      NodeValidator.checkNoNullElements(anonymousClassExpr.methods(), "methods", contextInfo);
+      NodeValidator.checkNoNullElements(anonymousClassExpr.statements(), "statements", contextInfo);
+
       for (MethodDefinition method : anonymousClassExpr.methods()) {
-        // 2. Static methods are not allowed in anonymous class.
+        // 3. Static methods are not allowed in anonymous class.
         Preconditions.checkState(!method.isStatic(), "Anonymous class cannot have static methods.");
-        // 3. Anonymous class cannot have explicit constructors.
+        // 4. Anonymous class cannot have explicit constructors.
         Preconditions.checkState(
             !method.isConstructor(), "Anonymous class cannot have explicit constructors.");
       }
-      // 4. Static variable expression is not allowed unless it is final.
+      // 5. Static variable expression is not allowed unless it is final.
       for (Statement statement : anonymousClassExpr.statements()) {
         if (statement instanceof ExprStatement) {
           Expr expr = ((ExprStatement) statement).expression();