Avoid crash when InsetBibitem::updateCommand shows a dialog

This is one of the places where a dialog is shown (indicating that
some bibtems have been renamed) while the metrics are not up to date.
Then a draw operation can be triggered too early and a crash can ensue.

Use ad-hoc solution for this case and use Buffer::setBusy(). The
Alert::warning helper cannot really do that since it doe snot know the
current buffer.

src/insets/InsetBibitem.cpp

@@ -95,9 +95,11 @@ void InsetBibitem::updateCommand(docstring const & new_key, bool)
 			++i;
 			key = new_key + '-' + convert<docstring>(i);
 		}
+		buffer().setBusy(true);
 		frontend::Alert::warning(_("Keys must be unique!"),
 			bformat(_("The key %1$s already exists,\n"
 			"it will be changed to %2$s."), new_key, key));
+		buffer().setBusy(false);
 	}
 	setParam("key", key);
 	buffer().invalidateBibinfoCache();