Cannot read property 'contentWindow' of null

[StephenEsser]

Issue Description

The sandbox iframe can be unmounted when a request is made to find its content window.

https://github.com/cerner/kaiju/blob/master/rails/client/app/bundles/kaiju/components/Project/utilities/messenger.js#L4

Error

Cannot read property 'contentWindow' of null

Issue Type

• New Feature
• Enhancement
• Bug
• Other

Expected Behavior

The Sandbox iframe should not be requested when it is unavailable.

Current Behavior

A request to fetch the Sandbox iframe is made when it is not present on the screen.

Steps to Reproduce

N/A