Clean up widgets in plot_view layout correctly

Iterating over the widgets in the layout in forward order when
attempting to delete all of the widgets in a layout resulted in
some widgets not getting deleted properly. Instead, they should
be interated over in reverse order so that the indices for each
widget do not change and all are deleted.

Fixes #5

Signed-off-by: Michael Jeronimo <michael.jeronimo@openrobotics.org>

rqt_bag/src/rqt_bag/timeline_menu.py

@@ -78,10 +78,10 @@ def show(self, context):
                 self._timeline.remove_listener(self._topic, self._viewer)
                 self._viewer = None
 
-            # clean out the layout
-            while self.layout().count() > 0:
-                item = self.layout().itemAt(0)
-                self.layout().removeItem(item)
+            # Clean out the layout. Loop backwards since removing
+            # from the beginning shifts items and changes the order
+            for i in reversed(range(self.layout().count())):
+                self.layout().itemAt(i).widget().setParent(None)
 
             # create a new viewer
             self._viewer = self._viewer_type(self._timeline, self, self._topic)