Unable to edit theme StyleBoxTexture

[me2beats]

Godot version:

3.2.3, 3.3rc7

OS/device including version:

Linux

Issue description:

Can't edit StyleBoxTexture after I add the theme to a control.

styleboxtexture.mp4

If I open the theme separately from a control (for example double clicking on the theme file in filesystem), this weird behavior doesn't happens.

Steps to reproduce:

1. Create a control (I created HSlider)
2. Add a theme on it
3. Add a StyleBoxTexture to an item (for example for HSlider it's slider, grabber area, grabber area highlight)
4. click on it to edit StyleBoxTexture parameters
   you will see the behavior shown in the video (above)

Minimal reproduction project: