call: fix possible crash upon call termination

Change-Id: I7ae9d60ddfbc248158154f3b8d78480601dd4b86
diff --git a/Ring/Ring/Calls/CallViewController.swift b/Ring/Ring/Calls/CallViewController.swift
index 2be2838..c34de13 100644
--- a/Ring/Ring/Calls/CallViewController.swift
+++ b/Ring/Ring/Calls/CallViewController.swift
@@ -449,7 +449,7 @@
         self.viewModel.dismisVC
             .observe(on: MainScheduler.instance)
             .subscribe(onNext: { [weak self] dismiss in
-                if dismiss {
+                if dismiss && self?.pipController != nil {
                     if #available(iOS 15.0, *) {
                         self?.pipController.stopPictureInPicture()
                         self?.pipController = nil