ID 480193 - Non-Enableable parameters in global Graph State Variables prevents renders and logs event handler error

Problem summary
Non-Enableable parameters as part of global Graph State Variables, GSVs, prevents renders from starting and logs an event handler error.

Customer reported version
katana.4.0v1

Customer reported platform
centos7

Steps to reproduce
1) Open the hello_pony.katana example project:
Help > Example Projects > Hello Pony > Open Project

2) In the Python tab, create a new global GSV as a standard parameter:

param = NodegraphAPI.GetRootNode().getParameter('variables')
param.createChildNumber('foo', 1)

3) Preview Render the GafferThree node.

Expected behavior
The render should start.

Actual behavior
The render does not start; nothing is logged in the Render Log tab.
The following event handler error:

[ERROR python.Utils.EventModule]: Error in event handler __on_event_idle() for event type "event_idle": AttributeError: 'NoneType' object has no attribute 'getValue'

Workaround
Convert the standard parameter to an enableable parameter in the global GSVs:

param = NodegraphAPI.GetRootNode().getParameter('variables')
param.deleteChild(param.getChild('foo'))
foo = param.createChildGroup('foo')
foo.createChildNumber('enable', 1)
foo.createChildNumber('value', 1)
foo.createChildNumber('default', 0)

Reproduced by support
This bug has been reproduced in:
Katana 4.0v4 - CentOS 7
Katana 4.0v3 - CentOS 7
Katana 4.0v2 - CentOS 7
Katana 4.0v1 - CentOS 7

Unable to reproduce bug in:
Katana 3.6v4 - CentOS 7
Katana 3.6v1 - CentOS 7
Katana 3.5v4 - CentOS 7
Katana 3.5v1 - CentOS 7
Katana 3.2v6 - CentOS 7
Katana 3.1v1 - CentOS 7
Katana 3.0v1 - CentOS 7
Katana 2.5v1 - CentOS 7
Katana 2.0v1 - CentOS 7

Earliest version tested
Katana 2.0v1 - This issue appears to be in all versions of the product