Adding a BitmapFont as a Custom Font in Project Settings causes errors

[ReanthSoft]

Godot version

3.4

System information

Windows 10, NVIDIA GeForce RTX 2080, GLES3

Issue description

In Godot 3.4, adding a BitmapFont as a Custom Font in the Project Setting's gui/theme/custom_font section causes errors when reopening the project. The same happens if a BitmapFont is added to a Theme which is then used in the gui/theme/custom section. This works in Godot 3.3.4

Steps to reproduce

Open the provided project and see the errors in the console.

Minimal reproduction project

BitmapFontBug.zip

[Calinou]

Related to #36614. See #36614 (comment) for a workaround.

[ReanthSoft]

Thanks, I've added the theme to a root Control node as a workaround.

[YuriSizov]

This is actually the same issue as #54752, which will be fixed by #55483. The problem is not with fonts, it's with loading images.