[activiti-webapp-explorer] disable post event when no task is active

[olegpavliv]

If
- I create a New Task in the explorer
- complete it
- post an event

Then an error message is shown with a stack trace in the log (task with a given id does not exist)

The Post Event should be enabled only when there is an active task.

I attached the files from the activiti-explorer module with necessary corrections.

[jbarrez]

That sounds like a bug indeed. Could you wrap your changes in a PR (https://github.com/Activiti/Activiti/pulls)?
that makes it easier for us, and you get the credit for the change!