Scheduler fix2015

## Issue

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5593

Essentially, a `NullReferenceException` was being thrown from line 69 of DynamoRevit.cs as the `revitDynamoModel` had been set to null.

## Stop-gap

Simply provide a valid reference of `RevitDynamoModel` as one of the arguments to `ExecuteOnIdleAsync`.  All usages of this method are also updated.