Crash on PIE exit after dynamically enabling and disabling tick functions through the Level Script blueprint.

User reports hitting an assertion due to invalid FTickTaskManager state when game is attempting to unregister a tick function that was previously disabled dynamically.

Failed assertion is in FTickTaskLevel::RemoveTickFunction(), in the 'FTickFunction::ETickState::Enabled' case:

else
{
	verify(AllEnabledTickFunctions.Remove(TickFunction) == 1); // otherwise you changed TickState while the tick function was registered. Call SetTickFunctionEnable instead.
}

1. Launch the editor and open the project contained in the attached archive (it's a 4.18 project, so choose to open a copy in later versions of the editor).
2. Double-click the 'TickTestMap' asset to open it.
3. PIE, and wait for the "Disabled Tick" message to appear in the log/console.
4. Press ESC (or click Stop) to attempt to exit PIE. A crash will result due to a failed assertion:

   [2018.03.23-15.43.11:907][  7]LogWindows: Error: === Critical error: ===
   [2018.03.23-15.43.11:909][  7]LogWindows: Error: 
   [2018.03.23-15.43.11:910][  7]LogWindows: Error: Assertion failed: AllEnabledTickFunctions.Remove(TickFunction) == 1 [File:D:\dev\Epic\UE4\Dev-Framework\Engine\Source\Runtime\Engine\Private\TickTaskManager.cpp] [Line: 1202] 
   [2018.03.23-15.43.11:912][  7]LogWindows: Error: 
   [2018.03.23-15.43.11:914][  7]LogWindows: Error: 
   [2018.03.23-15.43.11:916][  7]LogWindows: Error: 
   [2018.03.23-15.43.11:918][  7]LogWindows: Error: 
   

UE4Editor-Core-Win64-Debug.dll!FDebug::AssertFailed(const char * Expr, const char * File, int Line, const wchar_t * Format, ...) Line 419 C++
UE4Editor-Engine-Win64-Debug.dll!FTickTaskLevel::RemoveTickFunction(FTickFunction * TickFunction) Line 1204 C++
UE4Editor-Engine-Win64-Debug.dll!FTickTaskManager::RemoveTickFunction(FTickFunction * TickFunction) Line 1518 C++
UE4Editor-Engine-Win64-Debug.dll!FTickFunction::UnRegisterTickFunction() Line 1655 C++
UE4Editor-Engine-Win64-Debug.dll!FTickFunction::~FTickFunction() Line 1623 C++
[External Code]
UE4Editor-CoreUObject-Win64-Debug.dll!IncrementalPurgeGarbage(bool bUseTimeLimit, float TimeLimit) Line 1121 C++
UE4Editor-CoreUObject-Win64-Debug.dll!CollectGarbageInternal(EObjectFlags KeepFlags, bool bPerformFullPurge) Line 1516 C++
UE4Editor-CoreUObject-Win64-Debug.dll!CollectGarbage(EObjectFlags KeepFlags, bool bPerformFullPurge) Line 1540 C++
UE4Editor-UnrealEd-Win64-Debug.dll!UEditorEngine::EndPlayMap() Line 468 C++
UE4Editor-UnrealEd-Win64-Debug.dll!UEditorEngine::Tick(float DeltaSeconds, bool bIdleMode) Line 1924 C++
UE4Editor-UnrealEd-Win64-Debug.dll!UUnrealEdEngine::Tick(float DeltaSeconds, bool bIdleMode) Line 401 C++
UE4Editor-Win64-Debug.exe!FEngineLoop::Tick() Line 3382 C++
UE4Editor-Win64-Debug.exe!EngineTick() Line 63 C++
UE4Editor-Win64-Debug.exe!GuardedMain(const wchar_t * CmdLine, HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, int nCmdShow) Line 169 C++
UE4Editor-Win64-Debug.exe!WinMain(HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, char * __formal, int nCmdShow) Line 199 C++

Fix UE-51650

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