Fix potential crash in T3656 workaround

* src/windowmessages.cpp (CLOSE): Use the currentViewRef trick
to avoid unload in close.

The workaround codepath crashed reliably for me with the
async sending. But with the current item ref trick it no
longer crashes as the unload is later. My observation is
that any Invoke that triggers an Unload can crash so we
better avoid it.

• GnuPG-Bug-Id: T3656