Setting trendline name in Chart fails with "Error saving workbook: 'str' object has no attribute 'to_tree"

This was an issue may be a bug.
Raised in Stackoverflow Openpyxl Post "how to use openpyxl modify trendline name".
In attempting to answer the question this what I found.

Tested with the latest release 3.1.2 and all Line Numbers quoted refer to that release.

When creating a Chart (Scatter in this case) and setting the name of a trendline for the Chart an attribute error is returned when the workbook is saved. The name for the trendline is '123'.
"Error saving workbook: 'str' object has no attribute 'to_tree'

Reason; The error comes from the serialisable.py to_tree function @ line 106

When the 'tagname' is (str) 'trendline'
and the item being processed is (str) 'name' the following variables are set; line 128: child_tag = (str) 'name' line 130: obj = (str) '123'

At line 149 the child_tag 'name' is checked but fails because 'name' is not in self.__nested__
line 149: if child_tag in self.__nested__:

self.__nested__ contains other parameters that can be set like 'dispEq', 'dispRSqr', 'forward' etc but not name.

Code then drops to line 154
line 154: node = obj.to_tree(child_tag)

Given that 'obj' is a string for these values the line fails with the error;
AttributeError: 'str' object has no attribute 'to_tree'

As a test I edited trendline.py to change 'name' to a NestedString and this did rectify the issue for the provided sample code.