MATLAB Online App Designer help - WARNING: Error in subscription handler java.lang.​NullPointe​rException

Hey everyone,

Trolled these forums for years without asking a question, but I'm finally properly stuck!

I've been developing an app in App Designer for the past month or two. I've been working across my Mac (latest OSX, MATLAB 2019a) and a Windows (win10, MATLAB Online).

All has been running smoothly until last week when I was working on the Windows machine and was kindly greeted by the following when trying to launch app designer:

Sep 16, 2019 11:46:35 PM com.mathworks.connector.message_service.impl.MessageServiceJSONImpl handlePublishRequest
WARNING: Error in subscription handler
java.lang.NullPointerException
	at com.mathworks.ddux.JSBridge$1.handle(JSBridge.java:58)
	at com.mathworks.connector.message_service.impl.MessageServiceJSONImpl.handlePublishRequest(MessageServiceJSONImpl.java:29)
	at com.mathworks.connector.message_service.impl.AbstractMessageService.handlePublishRequest(AbstractMessageService.java:58)
	at com.mathworks.connector.message_service.impl.JniMessageServiceAdaptorImpl.doHandlePublish(JniMessageServiceAdaptorImpl.java:59)
	at com.mathworks.connector.message_service.api.JniMessageServiceAdaptor.fromNative(JniMessageServiceAdaptor.java:21)

What then happens is that App designer does launch in the browser, but when I try to open my project app designer doesnt change from the normal home page.

I read some solutions in other forum posts ascociated with a similar error. These are:

1. Logging out and back in again. Didnt help.
2. Waiting and trying later as it could be that the servers are down. Didnt help.
3. Reinstalling Java. Cant do this as its a corporate machine and I dont have admin privelages.

So I'm hoping someone has come across this and might have a solution?