I noticed that if my pipeline gets stopped, the glsinkbin displays some random data from graphicd memory instead of the last image. To prevent this I changed the behaviour in the way that the current buffer is set to NULL in transition from READY to NULL instead of PAUSED to READY.
At least that fixes the visual aspect. But I am not sure if that has any side effects I didnt notice yet.

Below the diff output for gstqtsink.cc:
---------------------------

index b6a6fa5..5471369 100644
--- a/ext/qt/gstqtsink.cc
+++ b/ext/qt/gstqtsink.cc
@@ -354,9 +354,9 @@ gst_qt_sink_change_state (GstElement * element, GstStateChange transition)
     case GST_STATE_CHANGE_PLAYING_TO_PAUSED:
       break;
     case GST_STATE_CHANGE_PAUSED_TO_READY:
-      qt_item_set_buffer (qt_sink->widget, NULL);
       break;
     case GST_STATE_CHANGE_READY_TO_NULL:
+        qt_item_set_buffer (qt_sink->widget, NULL);
       break;
     default:
       break;
diff --git a/ext/qt/qtitem.cc b/ext/qt/qtitem.cc