Use SignalContext instead of SignalWindow

ts/windows/init.ts

@@ -1,4 +1,8 @@
 // Copyright 2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
-window.SignalWindow.renderWindow();
+if (window.SignalContext.renderWindow) {
+  window.SignalContext.renderWindow();
+} else {
+  window.SignalContext.log.error('renderWindow is undefined!');
+}