Parent tk StringVar to the canvas widget, not to the toolbar.

... because the toolbar may be a fake object (when using
rcParams["toolbar"] = "toolmanager").

Try with `rcParams["toolbar"] = "toolmanager; use("tkagg")` and
interactively saving the picture.  No automated tests, sorry.

Author: anntzer

lib/matplotlib/backends/_backend_tk.py

@@ -843,7 +843,7 @@ def save_figure(self, *args):
 
         default_extension = self.canvas.get_default_filetype()
         default_filetype = self.canvas.get_supported_filetypes()[default_extension]
-        filetype_variable = tk.StringVar(self, default_filetype)
+        filetype_variable = tk.StringVar(self.canvas.get_tk_widget(), default_filetype)
 
         # adding a default extension seems to break the
         # asksaveasfilename dialog when you choose various save types