fix: My Courses screen detached fragment issue for ViewModel

- App crashes when the learner receives a delayed response from
  Firebase Remote Config on a detached 'MyCourseListFragment' and
  tries to initialize the viewModel on it. We will restrict the app
  from initializing ViewModel on a detached fragment to fix this
  issue.

Fixes: LEARNER-8986

OpenEdXMobile/src/main/java/org/edx/mobile/view/MyCoursesListFragment.kt

@@ -153,7 +153,7 @@ class MyCoursesListFragment : OfflineSupportBaseFragment(), RefreshListener {
     }
 
     private fun initInAppPurchaseSetup() {
-        if (environment.remoteFeaturePrefs.isValuePropEnabled()) {
+        if (isAdded && environment.remoteFeaturePrefs.isValuePropEnabled()) {
             initFullscreenLoader()
             initObservers()
         }