ttk: fix LabeledScale and OptionMenu destroy() method (#3026)

bpo-31135: Call the parent destroy() method even if the used
attribute doesn't exist.

The LabeledScale.destroy() method now also explicitly clears label
and scale attributes to help the garbage collector to destroy all
widgets.

Lib/lib-tk/ttk.py

@@ -1522,6 +1522,8 @@ class LabeledScale(Frame, object):
         else:
             del self._variable
         Frame.destroy(self)
+        self.label = None
+        self.scale = None
 
 
     def _adjust(self, *args):
@@ -1620,5 +1622,8 @@ class OptionMenu(Menubutton):
 
     def destroy(self):
         """Destroy this widget and its associated variable."""
+        try:
             del self._variable
+        except AttributeError:
+            pass
         Menubutton.destroy(self)

Misc/NEWS.d/next/Library/2017-08-08-14-59-26.bpo-31135.9q1QdB.rst

+ttk: Fix LabeledScale and OptionMenu destroy() method. Call the parent
+destroy() method even if the used attribute doesn't exist. The
+LabeledScale.destroy() method now also explicitly clears label and scale
+attributes to help the garbage collector to destroy all widgets.