When a mesh datablock is pinned in the properties panel,
`context.object` is `None`. This in turn causes `obj.mode` to raise an
`AttributeError` exception as `None.mode` doesn't exist.
Since there is no (fast/simple) way to check whether the owning object
is in edit mode or not, the properties will be disabled. Not ideal, but
better than spewing an exception on every panel draw.
Reviewed By: campbellbarton, brecht
Differential Revision: https://developer.blender.org/D5237
325501247d