GUI: add more debug output
fix exc in the graph, while vertex is being removed

java.lang.NullPointerException
null
lcmc.gui.DrbdGraph.getSource(DrbdGraph.java:616)
lcmc.gui.resources.DrbdVolumeInfo$8.action(DrbdVolumeInfo.java:429)
lcmc.gui.ClusterBrowser$DRBDMenuItemCallback.mouseOver(ClusterBrowser.java:2314)
lcmc.gui.resources.Info$11$1.run(Info.java:756)
fix possible npe in multi selection