WorldPartitionConvertCommandlet hits an assert when converting a persistent level that contains a sub-level with a landscape

Context:
The Window tab allows a user to open the Levels window in order to add a sub-level to a persistent level. For Unreal Engine 5.0 and beyond, World Partition has made the Levels window obsolete, and one can convert a level to world partition using Convert Level in the Tools tab.

Problem:
When attempting to use the Convert Level tool (WorldPartitionConvertCommandlet) on a persistent level which has a sub-level in it that contains a landscape, the conversion will fail. An assert will be hit on line 250 within ActorPartitionSubsystem.cpp. When using FLandscapeConfigHelper::PartitionLandscape, it will call FActorPartitionWorldPartition::GetActor from FindOrAddLandscapeStreamingProxy which will attempt to get a valid UWorldPartition for the sublevel in which the landscape exists and fail.

Expected:
The world partition conversion should succeed. The persistent level should be separated into grid cells, and the actors and levels within it should be automatically streamed accordingly based on streaming sources.

Repro project (Internal only):
1. Open attached project
2. Open the "Level1" level
3. Navigate to Window>Levels to open the Levels window, and note the sub-level "StreamingLevel1" (which contains the landscape actor)
4. Navigate to the Tools tab, and select "Convert level..."
5. Select "Level1", and hit Ok on the next menu with default settings
6. Observe: The conversion process fails, showing an assert in the output log
7. Expected: The conversion should succeed

Fresh project:
1. Create a new level "PersistentLevel"
2. Create another level "SubLevel"
3. Open "SubLevel", and switch to Landscape editor mode
4. On the New landscape menu that opens, click "Create" to add a landscape to this level, then save the level
5. Open "PersistentLevel", and navigate to the Window tab. Select "Levels" to open the levels window.
6. In the top-left of the levels window, click "Levels" to open a drop-down, select "Add Existing...", and choose to open "SubLevel"
7. Navigate to the Tools tab, and select "Convert level..."
8. Select "PersistentLevel" with default conversion settings, and click Ok on the next menu to begin the conversion
9. Observe: The conversion process fails, showing an assert in the output log
10. Expected: The conversion process should succeed

[2025.10.27-11.37.42:538][ 0]LogWorldPartitionConvertCommandlet: Display: PartitionLandscape started...
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: === Critical error: ===
[2025.10.27-11.39.10:021][ 0]LogWindows: Error:
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: Assertion failed: WorldPartition [...\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp] [Line: 250]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error:
[2025.10.27-11.39.10:021][ 0]LogWindows: Error:
[2025.10.27-11.39.10:021][ 0]LogWindows: Error:
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: Unhandled Exception: EXCEPTION_ACCESS_VIOLATION reading address 0x00000000000000a0
[2025.10.27-11.39.10:021][ 0]LogWindows: Error:
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6cc02a57 UnrealEditor-Engine.dll!TActorDescContainerInstanceCollection<TObjectPtr<UActorDescContainerInstance> >::TBaseIterator<1,AActor>::TBaseIterator<1,AActor><UWorldPartition>() [...\Engine\Source\Runtime\Engine\Public\WorldPartition\ActorDescContainerInstanceCollection.h:109]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6cc67903 UnrealEditor-Engine.dll!FWorldPartitionHelpers::ForEachActorDescInstance() [...\Engine\Source\Runtime\Engine\Private\WorldPartition\WorldPartitionHelpers.cpp:121]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6a7d1199 UnrealEditor-Engine.dll!FActorPartitionWorldPartition::GetActor() [...\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp:262]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6a7d164a UnrealEditor-Engine.dll!UActorPartitionSubsystem::GetActor() [...\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp:431]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6a7d1b41 UnrealEditor-Engine.dll!UActorPartitionSubsystem::GetActor() [...\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp:420]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc5c3effbe UnrealEditor-Landscape.dll!FLandscapeConfigHelper::FindOrAddLandscapeStreamingProxy() [...\Engine\Source\Runtime\Landscape\Private\LandscapeConfigHelper.cpp:88]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc5c3d3f82 UnrealEditor-Landscape.dll!`FLandscapeConfigHelper::ChangeGridSize'::`2'::<lambda_2>::operator()() [...\Engine\Source\Runtime\Landscape\Private\LandscapeConfigHelper.cpp:160]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6a7ce957 UnrealEditor-Engine.dll!FActorPartitionGridHelper::ForEachIntersectingCell() [...\Engine\Source\Runtime\Engine\Private\ActorPartition\ActorPartitionSubsystem.cpp:100]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc5c3df331 UnrealEditor-Landscape.dll!FLandscapeConfigHelper::ChangeGridSize() [...\Engine\Source\Runtime\Landscape\Private\LandscapeConfigHelper.cpp:226]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc5c414527 UnrealEditor-Landscape.dll!FLandscapeConfigHelper::PartitionLandscape() [...\Engine\Source\Runtime\Landscape\Private\LandscapeConfigHelper.cpp:296]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc667e32dd UnrealEditor-UnrealEd.dll!`UWorldPartitionConvertCommandlet::Main'::`2'::<lambda_8>::operator()() [...\Engine\Source\Editor\UnrealEd\Private\Commandlets\WorldPartitionConvertCommandlet.cpp:1076]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffc6682f032 UnrealEditor-UnrealEd.dll!UWorldPartitionConvertCommandlet::Main() [...\Engine\Source\Editor\UnrealEd\Private\Commandlets\WorldPartitionConvertCommandlet.cpp:1285]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ff653678fa0 UnrealEditor.exe!FEngineLoop::PreInitPostStartupScreen() [...\Engine\Source\Runtime\Launch\Private\LaunchEngineLoop.cpp:3860]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ff65366e39c UnrealEditor.exe!GuardedMain() [...\Engine\Source\Runtime\Launch\Private\Launch.cpp:143]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ff65366e6ba UnrealEditor.exe!GuardedMainWrapper() [...\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:128]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ff65367209e UnrealEditor.exe!LaunchWindowsStartup() [...\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:282]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ff653684e44 UnrealEditor.exe!WinMain() [...\Engine\Source\Runtime\Launch\Private\Windows\LaunchWindows.cpp:339]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ff6536880fa UnrealEditor.exe!__scrt_common_main_seh() [...\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288]
[2025.10.27-11.39.10:021][ 0]LogWindows: Error: [Callstack] 0x00007ffd78dde8d7 KERNEL32.DLL!UnknownFunction []