Describe the bug
When I click on the + button to join/create a MUC room and I fill the input text for the name of the room (for example "foo"), the join button does not work. I use a non-federated private XMPP server (ejabberd).

To Reproduce
Steps to reproduce the behavior:

1. Click on '+' button
2. Enter a room name
3. The "Join" remains disabled
4. See error

Expected behavior
I should be able to click on the Join button and create and join the room (on the default conference MUC service).

Environment (please complete the following information):

• Desktop
• Browser: Firefox
• Converse.js version: 10.1.7 (view_mode: "embedded")

Additional context
I tried on the master branch and even though the CSS for the embedded view has several issues, after tweeking it to be able to open the "Enter a new Groupchat" dialog, there it works. The Join button is not disabled and I can create and join the room "foo".
I'm not sure if you were aware that this feature was not working in the current last release of converse.js and I haven't find yet which commit fixed it in the master branch.