Activation of environments is broken in VSC Insiders.
The configuration setting terminal.integrated.shell.<platform> returns null in insiders.

See below:

• Operating System: Mac
• Works in Stable
• Does not work in Insiders

Version 1.34.0 (1.34.0)

Version 1.35.0-insider (1.35.0-insider) 2019-05-23T05:07:54.582Z