Avoid accessing vsWorker of undefined / null value

[jjyyxx]

According to microsoft/monaco-editor#766, I made small changes to the code. The message might be null if serializedMessage === 'null' or undefined if the JSON.parse throws a SyntaxError. So the code should check the nullity first.

Monaco editor itself, I think, will not make invalid messages like this. But other message source(like webpack) which is not intended for monaco editor may lead to this error.

[alexdima]

Thank you!