Fixed UninitializedPropertyAccessException in Custom Tab. (#4657)

Task/Issue URL:
https://app.asana.com/0/1200581511062568/1207587829937570/f

### Description
Fixed UninitializedPropertyAccessException in Custom Tab.

### Steps to test this PR
Note: Could not reproduce the issue.

### NO UI changes

Author: anikiki

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

@@ -2948,7 +2948,7 @@ class BrowserTabFragment :
         pulseAnimation.stop()
         animatorHelper.removeListener()
         supervisorJob.cancel()
-        popupMenu.dismiss()
+        if (::popupMenu.isInitialized) popupMenu.dismiss()
         loginDetectionDialog?.dismiss()
         automaticFireproofDialog?.dismiss()
         browserAutofill.removeJsInterface()