Fix Plot matrix view right click by checking SelSource is not null
NewSelectionSourceFromSelection can sometimes return NULL, We need to check the output of the method before using it. http://www.paraview.org/Bug/view.php?id=16055
NewSelectionSourceFromSelection can sometimes return NULL, We need to check the output of the method before using it. http://www.paraview.org/Bug/view.php?id=16055