Check activity != null before accessing it

Test: Manually
Bug: 404543904
Change-Id: Id63555f57f5c082c1cb7f49afefaf776251b6190
diff --git a/android/TerminalApp/java/com/android/virtualization/terminal/TerminalTabFragment.kt b/android/TerminalApp/java/com/android/virtualization/terminal/TerminalTabFragment.kt
index a0c6e4e..36f939c 100644
--- a/android/TerminalApp/java/com/android/virtualization/terminal/TerminalTabFragment.kt
+++ b/android/TerminalApp/java/com/android/virtualization/terminal/TerminalTabFragment.kt
@@ -201,7 +201,7 @@
     }
 
     private fun updateMainActivity() {
-        val mainActivity = (activity as MainActivity)
+        val mainActivity = activity as MainActivity ?: return
         if (terminalGuiSupport()) {
             mainActivity.displayMenu!!.visibility = View.VISIBLE
             mainActivity.displayMenu!!.isEnabled = true