From what I understand, the engine cannot find the moduleprefix of the game, because it wants to load the module prefix from the current executable (which is the editor) and thus, it doesn’t find a path. Consequently, it does not fill the Module Structure and ends up with an empty record for the game module.
For reference, look at the file ModuleManager.cpp. I think the FindModulePathsInDirectory might have an issue…
Please note that I have zero experience with the engine, so I might be wrong.