Editor: GUIStyle lost on Project Run?

Sorry if this is in the wrong place, not sure where to post for creating custom editor code.

I have created a custom editor and set a Texture as a GUIStyle for a box


However when hitting the project's Run button and then stopping it, all the GUIStyles are lost on my editor:


I've done 3 custom editors so far, and all have the same problem. All GUIStyles lost, to the point where I had to make a button on them to recreate the textures and styles.

How can I make it keep the styles?
(That waveform needs to be generated, so it takes a minute every time just to get the image back, very frustrating)

Thanks!