At this stage, the conversion

sage: M = Manifold(2, 'M')
sage: M.diff_form_module(1)(1)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
...
AttributeError: 'NoneType' object has no attribute '_domain'

fails with an AttributeError.

This should rather yield a NotImplementedError or TypeError with the message that there is no conversion available.

CC: @egourgoulhon @tscrim @mkoeppe

Component: manifolds

Author: Michael Jung

Branch/Commit: 28dec69

Reviewer: Travis Scrimshaw, Eric Gourgoulhon

Issue created by migration from https://trac.sagemath.org/ticket/30191