Lock timeline when deleting widgets...

Timeline/Sequence.C

@@ -414,7 +414,11 @@ Sequence::handle ( int m )
                         Sequence_Widget::belowmouse( NULL );
                     }
 
+                    timeline->wrlock();
+
                     delete t;
+
+                    timeline->unlock();
                 }
 
                 Loggable::block_end();