Are you running from a source build engine? If so you could track down the fail point in the engine.
The bool ULandscapeEditorObject::SetAlphaTexture in the cpp file seems to be setting the alpha texture for the terrain and not the editor brush.
There was a change between 5.0.3 source and 5.1
5.1 added a check
// Load fallback if there's no texture or valid data
if (NewAlphaTexture == nullptr)
{
UTexture2D* DefaultAlphaTexture = GetClass()->GetDefaultObject<ULandscapeEditorObject>()->AlphaTexture;
check((DefaultAlphaTexture != nullptr) && DefaultAlphaTexture->Source.IsValid()); // The default texture should always be valid
//..... more code
}
This is the line that fails
UTexture2D* DefaultAlphaTexture = GetClass()->GetDefaultObject<ULandscapeEditorObject>()->AlphaTexture;
I’m guessing GetClass()->GetDefaultObject()->AlphaTexture; is the line that sinks the engine. It for some reason returns a nullptr even though it’s supposed to be failproof.
If you added an alpha channel by hand then maybe it would not trigger the crash. (not sure how to do that out of engine though)