[Canvas] Avoid null TransferHandler

Transfer handlers on Inspector labels
are not necessary after inspected view
is clear, as there is nothing to transfer.
However, Swing's event-handling does go
back to inform the original transfer
handler that an event has completed; if
this happens after the component's
transfer handler has been nullified,
nasa/mct#172 results.

To avoid this issue, instead of nullifying
the transfer handler, simply introduce
checks such that it becomes inert when
there is no view in the Inspector.
Merge pull request #278 from VWoeltjen/mct172

[Canvas] Avoid null TransferHandler