For me I was adding a store the stores: in both application.js and a controller. Once I removed out of one or the other the issue went away. Don't know if this is the same across the board but it worked for me.
Same problem with extjs 18.104.22.1683 : this error appears only with firefox.
I try to change the requires order, because when I set only one model in the Application config, it works if the class model is added first in the "requires" configuration.
I was having this problem on Firefox too. The problem turned out to be that I was specifying some 'controllers' in my application config, but I wasn't requiring them. The confusing part is that it would work occasionally, but more often it would not work. It seems there may be a race condition of sorts in the Ext.application function somewhere. I added my controllers to the 'requires' config and now it loads properly every time.
I'm guessing the same thing happens if you have 'stores' or 'models' configured, but do not 'require' them.