`prepare_cloned_fields()` should check for cloning support on model

[jeremystretch]

Deployment Type

NetBox Cloud

NetBox Version

v4.2.2

Python Version

3.10

Steps to Reproduce

1. Define a clone() method on a plugin model, returning None.
2. Install the plugin.
3. Attempt to view an instance of that model.

Expected Behavior

The object should render successfully.

Observed Behavior

An unhandled AttributeError exception is raised:

'NoneType' object has no attribute 'items'

This is because NetBox expects the clone() method, if defined on a model, to return a set of attributes suitable for pre-populating the creation form for the model. However, a plugin author might want to introduce a clone() method for unrelated reasons.

NetBox should inspect the model and attempt to call clone() only for models which inherit from CloningMixin.