It sounds to me like a phantom window problem. The game has code built in to create multiple chat windows, but its not complete & has a few bugs in it. MultiChatWindow turns that feature on & fixes some of the bugs. The extra chat windows are managed by the game, not any addon which is why the window your seeing still exists even though you removed the addons. The reason its causing errors is because, as I said, they didn't finish the code for it, & it doesn't work correctly, but MultiChat fixes those bugs. But since you removed MultiChat , its not there to fix them for you.
My guess is that if you reinstall MultiChatWindow and log in (you may have to log in then reload ui) it will probably fix the problem for you. If not you could install my
ChatIt addon which has the ability for MultiChatWindows, turn the multiChatWindow feature on in settings, reload the ui by typing "/reloadui" in the chat window and that should fix it.
If it still does not ChatIt has a built in ability to manually destroy all of the extra chat windows. In the settings menu there is a button that says "Destroy Chat Windows" ("Allow Multiple Chat Windows" must be turned on to use it). If you click that it will definitely get rid of those phantom windows for you. Then if you want you can turn "Allow Multiple Chat Windows" back off, uninstall ChatIt, & Reinstall MultiChatWindows.