Fix ViewModels IllegalStateException (#4720)

Task/Issue URL:
https://app.asana.com/0/1200204095367872/1207733727777585/f

### Description
Only executes tab hidden/shown logic if the fragment is attached.

### Steps to test this PR
- [ ] Code review.

app/src/main/java/com/duckduckgo/app/browser/BrowserTabFragment.kt

@@ -2924,12 +2924,14 @@ class BrowserTabFragment :
     }
 
     private fun onTabHidden() {
+        if (!isAdded) return
         viewModel.onViewHidden()
         downloadMessagesJob.cancel()
         webView?.onPause()
     }
 
     private fun onTabVisible() {
+        if (!isAdded) return
         webView?.onResume()
         launchDownloadMessagesJob()
         viewModel.onViewVisible()