Hi ,
I am glad to see that you were able to find a solution. I just wanted to provide some additional information about why this is necessary.
In order for the UnrealHeaderTool and UnrealBuildTool to recognize a USTRUCT, it must be contained within a header file that contains a UCLASS macro. As you noticed, without the UCLASS in place, the Editor crashed when opening the project. If you had selected any other option as the parent of your new class (eg. Object), the header file that would have been created for you would have automatically included the UCLASS macro and you would not have seen the crash.