Importing Camera FBX into a sequence can crash with a headless editor

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.

1. Create a sequence with a camera and animate it
2. Export the cameras animation to an FBX
3. Rename the camera
4. Create a python script that imports a the FBX back onto a sequence (see linked UDN)
5. Run that script in a headless editor (see linked UDN for example command line)

RESULT

The editor crashes trying the get the slate notification manager

1. [2021.03.17-12.53.24:916][ 0]LogOutputDevice: Warning:
2.  
3. Script Stack (1 frames):
4. SequencerToolsFunctionLibrary.ImportFBX
5.  
6. [2021.03.17-12.53.34:238][ 0]LogWindows: Error: === Critical error: ===
7. [2021.03.17-12.53.34:238][ 0]LogWindows: Error:
8. [2021.03.17-12.53.34:242][ 0]LogWindows: Error: Assertion failed: IsValid() [Link Removed] [Line: 890]
9. [2021.03.17-12.53.34:244][ 0]LogWindows: Error:
10. [2021.03.17-12.53.34:244][ 0]LogWindows: Error:
11. [2021.03.17-12.53.34:245][ 0]LogWindows: Error:
12. [2021.03.17-12.53.34:245][ 0]LogWindows: Error: [Callstack] 0x00007ff96a3ad759 KERNELBASE.dll!UnknownFunction []
13. [2021.03.17-12.53.34:246][ 0]LogWindows: Error: [Callstack] 0x00007ff927ab4be6 UE4Editor-Core.dll!ReportAssert() [d:\staging\github-unrealengine\engine\source\runtime\core\private\windows\windowsplatformcrashcontext.cpp:1627]
14. [2021.03.17-12.53.34:247][ 0]LogWindows: Error: [Callstack] 0x00007ff927ab8058 UE4Editor-Core.dll!FWindowsErrorOutputDevice::Serialize() [d:\staging\github-unrealengine\engine\source\runtime\core\private\windows\windowserroroutputdevice.cpp:79]
15. [2021.03.17-12.53.34:247][ 0]LogWindows: Error: [Callstack] 0x00007ff9277d26fd UE4Editor-Core.dll!FOutputDevice::LogfImpl() [d:\staging\github-unrealengine\engine\source\runtime\core\private\misc\outputdevice.cpp:61]
16. [2021.03.17-12.53.34:248][ 0]LogWindows: Error: [Callstack] 0x00007ff92776d5d7 UE4Editor-Core.dll!AssertFailedImplV() [d:\staging\github-unrealengine\engine\source\runtime\core\private\misc\assertionmacros.cpp:102]
17. [2021.03.17-12.53.34:249][ 0]LogWindows: Error: [Callstack] 0x00007ff92776f590 UE4Editor-Core.dll!FDebug::CheckVerifyFailedImpl() [d:\staging\github-unrealengine\engine\source\runtime\core\private\misc\assertionmacros.cpp:458]
18. [2021.03.17-12.53.34:256][ 0]LogWindows: Error: [Callstack] 0x00007ff908582ed6 UE4Editor-MovieSceneTools.dll!MovieSceneToolHelpers::ImportFBXCameraToExisting() [d:\staging\github-unrealengine\engine\source\editor\moviescenetools\private\moviescenetoolhelpers.cpp:2481]
19. [2021.03.17-12.53.34:257][ 0]LogWindows: Error: [Callstack] 0x00007ff8f5167d49 UE4Editor-SequencerScriptingEditor.dll!USequencerToolsFunctionLibrary::ImportFBX() [d:\staging\github-unrealengine\engine\plugins\moviescene\sequencerscripting\source\sequencerscriptingeditor\private\sequencertools.cpp:462]
20. [2021.03.17-12.53.34:259][ 0]LogWindows: Error: [Callstack] 0x00007ff8f516d7cd UE4Editor-SequencerScriptingEditor.dll!USequencerToolsFunctionLibrary::execImportFBX() [d:\staging\github-unrealengine\engine\plugins\moviescene\sequencerscripting\intermediate\build\win64\ue4editor\inc\sequencerscriptingeditor\sequencertools.gen.cpp:331]
21. [2021.03.17-12.53.34:262][ 0]LogWindows: Error: [Callstack] 0x00007ff926eb2d78 UE4Editor-CoreUObject.dll!UFunction::Invoke() [d:\staging\github-unrealengine\engine\source\runtime\coreuobject\private\uobject\class.cpp:5588]

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