Test the last castle in world 6. Once the key is grabbed after defeating the boss, the game freezes unless the DS/DSi is closed before grabbing the key.
I don't have a wii u anymore to test it myself, but if homebrews are not visible on the wii u menu I think you can press L + R + minus to open the plugin menu, there should be an option called "homebrews on wii u menu" or something similar