ID is not referenced in any style rules

after several hours of successful compiling and running my small app, i suddenly get these errors in my index.xml file:

"The id 'menu' is not referenced in any style ryles". I am not able to compile or run my app. I know that my android emulator works because I can run other apps on it. This is my console error:

[ERROR] Application Installer abnormal process termination. Process exit value was 1

This is my code in index.xml (yes, it's the tutorial)

<Menu id="menu" platform="android">
    <!-- Cannot specify node text.  Use attributes only. -->
    <MenuItem id="addBook"
        title="Add book"
        onClick="addBook"
    showAsAction="Ti.Android.SHOW_AS_ACTION_IF_ROOM" />
    </Menu>

And my index.tss

"menu":{
    backgroundColor:"white"
},
 
"addBook":{
    backgroundColor: "red"
}

I have the error on "menu" and on "addBook", even though they're in the stylesheet.

I tried cleaning the project, and I am able to recreate this project, and import others, that work just fine.