In a headless editor instance we will not be able to send toasts to the user, and we should not notify slate. The Sequencer Tools function library calls MovieSceneToolHelpers::ImportFBXCameraToExisting always passing true to bNotifySlate. This should be changed to account for cases when we cannot notify slate.
The editor crashes trying the get the slate notification manager
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-111594 in the post.