In Tabs.qml, indexOf() of repeaters is called when repeaters is (apparently) undefined. See below:
tim@trusty:~/dev/ubuntu-ui-toolkit/staging$ source export_modules_dir.sh
tim@trusty:~/dev/ubuntu-ui-toolkit/staging$ cd tests/resources/navigation/
tim@trusty:~/dev/ubuntu-ui-toolkit/staging/tests/resources/navigation$ qmlscene RepeaterTabs.qml
unity::action::ActionManager::ActionManager(QObject*):
Could not determine application identifier. HUD will not work properly.
Provide your application identifier in $APP_ID environment variable.
file:///home/tim/dev/ubuntu-ui-toolkit/staging/modules/Ubuntu/Components/Tabs.qml:342: TypeError: Cannot call method 'indexOf' of undefined
file:///home/tim/dev/ubuntu-ui-toolkit/staging/modules/Ubuntu/Components/Tabs.qml:342: TypeError: Cannot call method 'indexOf' of undefined
The same warning also shows when running the unit tests for the tabs.
It may have been fixed in the dynamic tabs MR, but that one was reverted.