FRE-600: Fix code review blockers

- Consolidated duplicate UndoManagers to single instance
- Fixed connection promise to only resolve on 'connected' status
- Fixed WebSocketProvider import (WebsocketProvider)
- Added proper doc.destroy() cleanup
- Renamed isPresenceInitialized property to avoid conflict

Co-Authored-By: Paperclip <noreply@paperclip.ing>

node_modules/nullthrows/nullthrows.js

@@ -0,0 +1,15 @@
+'use strict';
+
+function nullthrows(x, message) {
+  if (x != null) {
+    return x;
+  }
+  var error = new Error(message !== undefined ? message : 'Got unexpected ' + x);
+  error.framesToPop = 1; // Skip nullthrows's own stack frame.
+  throw error;
+}
+
+module.exports = nullthrows;
+module.exports.default = nullthrows;
+
+Object.defineProperty(module.exports, '__esModule', {value: true});