fix(rpc): ensure better error messages when rpc misbehaves (#3838)

- Print parentGuid when it is not available for __create__.
  Some bots show generic "something is undefined" error - let's
  get better information about the failure.

- Ignore events on disposed objects outside of tests.
  Some bots show this happening for "previewUpdated" - let's see
  whether there are more important events that misbehave.

src/client/connection.ts

@@ -133,7 +133,9 @@ export class Connection {
   }
 
   private _createRemoteObject(parentGuid: string, type: string, guid: string, initializer: any): any {
-    const parent = this._objects.get(parentGuid)!;
+    const parent = this._objects.get(parentGuid);
+    if (!parent)
+      throw new Error(`Cannot find parent object ${parentGuid} to create ${guid}`);
     let result: ChannelOwner<any, any>;
     initializer = this._replaceGuidsWithChannels(initializer);
     switch (type) {

src/dispatchers/dispatcher.ts

@@ -18,7 +18,7 @@ import { EventEmitter } from 'events';
 import * as channels from '../protocol/channels';
 import { serializeError } from '../protocol/serializers';
 import { createScheme, Validator, ValidationError } from '../protocol/validator';
-import { assert, createGuid, debugAssert } from '../utils/utils';
+import { assert, createGuid, debugAssert, isUnderTest } from '../utils/utils';
 
 export const dispatcherSymbol = Symbol('dispatcher');
 
@@ -75,6 +75,12 @@ export class Dispatcher<Type, Initializer> extends EventEmitter implements chann
   }
 
   _dispatchEvent(method: string, params: Dispatcher<any, any> | any = {}) {
+    if (this._disposed) {
+      if (isUnderTest())
+        throw new Error(`${this._guid} is sending "${method}" event after being disposed`);
+      // Just ignore this event outside of tests.
+      return;
+    }
     this._connection.sendMessageToClient(this._guid, method, params);
   }