Homogenised error handling in Invoke functions.
Homogenised error handling in Invoke functions.

Renamed method IsExiting to IsInvokable to better describe its contents.
Added null check in it (previously only one of the four Invoke methods was
checking for null).
Added null check when we set the TestExceptionString in automated test mode.
Added logging to exception handling.

Signed-off-by: Konstantina Chremmou <a7d1f757a659045fb6db8c1e3849a1c8ad00b97a@citrix.com>

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