Don't try to update the clear button state when there's no DCC window.

src/fe-gtk/dccgui.c

@@ -1128,6 +1128,7 @@ fe_dcc_update (struct DCC *dcc)
 		dcc_update_chat (dcc);
 	}
 
+	if (dccfwin.window)
 		update_clear_button_sensitivity();
 }