So, that guide has you enable the source menu in Wiiflow. Can you confirm that it's working? That is (different from the coverflow view) you have a screen with various consoles/types on it, and when you select one Wiiflow renders the coverflow for just that system. I think the default version always has Wii and Gamecube, so those two source menu buttons should be working.
Assuming all of the above is true, it's just that when you click on the retro console source menu buttons they come up blank, you may just need to configure the source menu. I use Maxternal's source menu editor available here
https://gbatemp.net/threads/maxternals-wiiflow-plugins-and-java-apps.344802/ . The short version is: the source menu button must be programmed, using the editor, to point to the correct Wiiflow plugin (i.e. FCEUGX).
That wiiflow plugin (located in (/wiiflow/plugins/), the .dol file, is defined by its corresponding ini, which refers to the appropriate rom directory, and of course you have to actually put the roms there.
FCEUGX crashing is almost certainly a separate issue from anything in Wiiflow.