Fix Compatible issues with Global Context in NodeRed 0.15.x

[ducminhquan]

Hope it helps.

[guidone]

Hi @ducminhquan thanks for the PR, I've solved in a different way on 0.6.7

Basically I've got rid of the node-red context, honestly I was not able to figure out why in some versions/installation the global context is not available. I'm saving the chat context in a private variable (in the future this chat context will be persisted somewhere), node-red contexts are just unreliable.

Thanks for your feedback and contribute, it's very much appreciated

Guido