It was broken by some changes since 4.18 . My quick fix for this is adding this line to DestructibleFractureSettings.cpp ( if you use C++ version of the Engine, the file should be inside UnrealEngine\Engine\Plugins\Runtime\ApexDestruction\Source\ApexDestruction\Private :
// For now just copy the submesh data from the 0 submesh.
apex::ExplicitSubmeshData SubmeshData;
apex::ExplicitSubmeshData* SourceSubmeshData = HMesh.submeshData(0);
if (SourceSubmeshData != NULL)
{
SubmeshData = *SourceSubmeshData;
}
FCStringAnsi::Strncpy(SubmeshData.mMaterialName, "InteriorMaterial", nvidia::apex::ExplicitSubmeshData::MaterialNameBufferSize);
// BRG - this is being done until I add the ability to reset the submesh data without resetting the mesh data in the fracture tools
if (HMesh.submeshCount() > (uint32)OriginalSubmeshCount)