Install the most recent version of MC18. Then you should be able to uninstall.
Remember to backup your MC19 library first!
EDIT: My apologies for my expressed verbiage (which you appropriately remove), but considering what happened to my MC19 library I'm sure you can understand my anger. In any case, just so that it shouldn't happen to the next guy, let me explain what happened ...
I took JimH's suggestion and installed the most recent version of MC18. When uninstalling, I chose the option that implies I was upgrading, and assumed MC19 would be untouched. Wrong! After uninstalling MC18, MC19 didn't know where its library was because I moved it from the user area, to an area that could be more easily address when backing up MC files. Therefore, and because I hadn't backed up my MC19 library, it had lost all media added since, as well as all database associations that would track "last played" (etc). All presentations of lists had lost their preferences and/or had to be redefined, etc, etc, ... and I'm still finding preferences needing to be redefined. In fact, I'm not totally sure restoring the library from a more recent backup would have restored anything other than database references to my media files ... i.e., I'm thinking all my MC19 preferences would have been lost(?)
It's almost as if installing MC18 stole many settings, leastwise re-establishing them as new. It would be interesting (for others) to know if this had turned out different if I had re-installed MC19 right after installing MC18, and before uninstalling 18(??)
Feeling better today