You may listen to import events to figure out when a diagram got imported.
To quote from the Viewer#importXML documentation:
During import the viewer will fire life-cycle events:
* import.parse.start (about to read model from xml)
* import.parse.complete (model read; may have worked or not)
* import.render.start (graphical import start)
* import.render.complete (graphical import finished)
* import.done (everything done)