change signature hierarchy: don't build children to check for leaf (IDEA-157682)

platform/lang-impl/src/com/intellij/refactoring/changeSignature/MethodNodeBase.java

@@ -85,6 +85,14 @@ public abstract class MethodNodeBase<M extends PsiElement> extends CheckedTreeNo
     return super.getChildCount();
   }
 
+  @Override
+  public boolean isLeaf() {
+    if (children == null) {
+      return false;
+    }
+    return super.isLeaf();
+  }
+
   @Override
   public int getIndex(TreeNode aChild) {
     buildChildren();