5.8 InstancedFoliageActor.add_instances crash editor

Development Programming & Scripting
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The problem is when enabling the Mesh Partition plugin in 5.8. Then use python to add an instance to a foliageactor. Then the editor will crash.

import unreal
w = unreal.get_editor_subsystem(unreal.UnrealEditorSubsystem).get_editor_world()
ft = unreal.EditorAssetLibrary.load_asset('/Game/Test/NewInstancedStaticMeshFoliage')
unreal.InstancedFoliageActor.add_instances(w, ft, [unreal.Transform()])
2

And here are analysis from Claude Code
[5.8] Foliage AddInstances() crashes when the MeshPartition plugin is enabled

Version: 5.8.1 | Platform: Win64 Editor | Repro: 100%

Repro

  1. New blank project, enable the MeshPartition plugin (Experimental), restart.
  2. Run in the Python console:
import unreal
w  = unreal.get_editor_subsystem(unreal.UnrealEditorSubsystem).get_editor_world()
ft = unreal.EditorAssetLibrary.load_asset('/Game/FT_Test')   # any UFoliageType
unreal.InstancedFoliageActor.add_instances(w, ft, [unreal.Transform()])

Editor crashes with EXCEPTION_ACCESS_VIOLATION reading address 0x10.
The Blueprint Add Instances node crashes identically. Disabling MeshPartition
fixes it.

Root cause

FFoliageInstanceBaseCache::AddInstanceBaseId() calls the new-in-5.8
IFoliageEditModuleBase hook before null-checking the component
(Runtime/Foliage/Private/FoliageInstanceBase.cpp:126):

if (IFoliageEditModuleBase* FoliageModule = IFoliageEditModuleBase::Get(); ensure(FoliageModule))
{
    bIgnoreComponentClass = FoliageModule->ShouldIgnoreComponentForBaseID(InComponent); // nullptr
}
#endif
if (InComponent && !InComponent->IsCreatedByConstructionScript() && !bIgnoreComponentClass) // too late

and the implementation dereferences it unguarded
(Editor/FoliageEdit/Private/FoliageEditModule.cpp:262):

for (TSoftObjectPtr<UClass> ClassToCheckPtr : ComponentBaseIDClassesToIgnore)
    if (UClass* ClassToCheck = ClassToCheckPtr.Get())
        if (InComponent->IsA(ClassToCheck))   // <-- AV, InComponent is null

AddInstanceImpl (InstancedFoliage.cpp:2281) passes null because
FFoliageInstances created by the AddInstances UFUNCTION have no
BaseComponent, and the API doesn’t expose one. Interactive brush painting
survives only because it has a real hit component.

The crash needs ComponentBaseIDClassesToIgnore to be non-empty, and the only
thing in the engine that populates it is
FMeshPartitionEditorModule::RegisterFoliageBaseIDClassIgnoreList()
(MeshPartitionEditor/Private/MeshPartitionEditorModule.cpp:143), which
registers three classes at startup. Hence: MeshPartition on = every scripted
foliage add crashes.

Regression

New in 5.8. 5.7 had no hook and null-checked first:
if (InComponent && !InComponent->IsCreatedByConstructionScript()).

Fix

One line in ShouldIgnoreComponentForBaseID:

if (!InComponent) { return false; }

(or guard the call site in AddInstanceBaseId — ideally both).