videowidget: fix the possible unreset pointer problem for distant and preview renderer

Change-Id: I3288aebe83c47258ae4dcbff18fe98440d96abb3

videowidget.cpp

@@ -397,12 +397,14 @@ void
 VideoWidget::slotStopFullView(const std::string& id)
 {
     QObject::disconnect(rendererConnections_.updated);
-    QObject::disconnect(rendererConnections_.stopped);
     using namespace lrc::api::video;
     if (id == PREVIEW_RENDERER_ID) {
         previewRenderer_ = nullptr;
     } else {
         distantRenderer_ = nullptr;
     }
+    if (!previewRenderer_ && !distantRenderer_) {
+        QObject::disconnect(rendererConnections_.stopped);
         repaint();
     }
+}