QML Debugger: Don't hang if there are no scopes to be retrieved

Qt 5.11.0 contains an optimization that does away with call contexts for
simple inline bindings. Also, it doesn't report QML contexts as scopes
in the "frame" and "backtrace" commands. Therefore, in those cases
"this" is the only thing to be retrieved.

Check if we are done when "this" has been retrieved and no scopes have
been found.

Change-Id: I9e0f545777bc38333938b65a934d42701ec4f807
Task-number: QTBUG-68218
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/qml/qmlengine.cpp

@@ -2167,6 +2167,10 @@ void QmlEnginePrivate::handleFrame(const QVariantMap &response)
         watchHandler->insertItem(item);
         evaluate(exp, -1, [this, iname, exp](const QVariantMap &response) {
             handleEvaluateExpression(response, iname, exp);
+
+            // If there are no scopes, "this" may be the only thing to look up.
+            if (currentFrameScopes.isEmpty())
+                checkForFinishedUpdate();
         });
     }