Keep cachedRenderer as None when pickling Figure.

anntzer · anntzer · commit 583e1d91b5cb · 2020-06-21T17:17:19.000+02:00
`Figure.__init__` sets cachedRenderer to None, so it's awkward that
pickling and unpickling a figure completely removes the attribute,
causing obscure AttributeErrors.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -2108,8 +2108,10 @@ def __getstate__(self):
         # The canvas cannot currently be pickled, but this has the benefit
         # of meaning that a figure can be detached from one canvas, and
         # re-attached to another.
-        for attr_to_pop in ('canvas', '_cachedRenderer'):
-            state.pop(attr_to_pop, None)
+        state.pop("canvas")
+
+        # Set cached renderer to None -- it can't be pickled.
+        state["_cachedRenderer"] = None
 
         # add version information to the state
         state['__mpl_version__'] = _mpl_version
