This problem is related to defect # DE511556
1.Create Email channel from Email page.
2.Create one template from message template page.
3.Email page, Link template what we created on message template page
4.Later delete the template what we have created on message template page.
5.If we open this linked Email channel in Email page it is breaking because we deleted that template on message template page.
DX Operational Intelligence 20.x
DX Application Performance Management 20.x
DX AXA 20.x
Defect fixed in DX 21.3