#678 NPE on click after root selection

[Nicolas Beauger]

Double click at the start of a document inthe Rodin editor, then click on another line. The following exception is thrown:

Thread [main] (Suspended (exception NullPointerException))
RodinOperationUtils.isReadOnly(ILElement) line: 191
SelectionController.canMoveSelection() line: 176
SelectionController.mouseDown(MouseEvent) line: 252
TypedListener.handleEvent(Event) line: 191
EventTable.sendEvent(Event) line: 84
StyledText(Widget).sendEvent(Event) line: 1053
...

[Nicolas Beauger]

Fixed by commit ea57e6b.