Debugger: Fix double clicking watch handler

Double clicking watch handler window without hitting an item
used to trigger 'Add New Expression Evaluator...'.

Change-Id: I452eb56476f7186ac36f7a9f1e9d2a43c109a9b1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/watchhandler.cpp

@@ -1056,7 +1056,7 @@ bool WatchModel::setData(const QModelIndex &idx, const QVariant &value, int role
         }
 
         if (ev.as<QMouseEvent>(QEvent::MouseButtonDblClick)) {
-            if (!item) {
+            if (!item->parent()) { // if item is the invisible root item
                 inputNewExpression();
                 return true;
             }