This looks to be a similar issue to : Blueprint compile fatal error, only using Print String test loop. - Audio - Unreal Engine Forums
I can reliably reproduce the issue with a new project.
- Create a custom event.
- Drag out to from custom event create a for loop
- Drag out for loop’s “index” and call “to Text (int)”
- Drag out “To texts” return value and call a “format text”
- Add a pin to “Format text”
- Drag a second pin from “To texts” return value to the newly created pin
- right click on “Format Text” first input pin and break the link to “to text”
- In “format Text” first pin (now a textbox as you broke the link) enter any text
- Click anywhere or compile and it will crash
To fix the issue you have to remove the custom event and the for loop. If you just remove the forloop you will continue to have problems despite it compiling.