Editor crash in EditorResourcePreview when assigned as fallback bitmap font

[qarmin]

Godot version: Godot 3.1 RC 2

OS/device including version: Linux Mint 19

Issue description: When assigning fallback of fallback to root BitmapFont or next pass in Material, then editor crash. EDIT: It happens also with Atlas Texture and probably others, but for now I don't know how to fix this.

• [x] - 1. Bitmap font and Material
• [ ] - 2. Textures

Steps to reproduce:

1. Create new resource Bitmap font
2. Save it
3. Open this Bitmap font
4. Assign to fallback new Bitmap font
5. To newly created Bitmap font assign as fallback previously saved font(root one)

https://streamable.com/gy4jp

[Anutrix]

How to reproduce this for atlas texture? Update: I was able to reproduce this in master d89dc5794. But not sure if this counts as fallback. Here's how to reproduce:

1. Right-click res:// in File System and create a new resource of type Atlas Texture named A.tres.
2. Click Atlas property of first and create a new Atlas Texture.
3. Click Atlas property of second and load A.tres.

[qarmin]