Crash generating grass for landscape with XY offset map

Landscape grass map generation crashes if the landscape component in question uses an XY offset map.

---------

Fatal error: [Link Removed] [Line: 646]
Rendering thread exception:
Fatal error: [Link Removed] [Line: 1493]

Couldn't find Shader FLandscapeGrassWeightPS for Material Resource M_LandscapeDISP!
With VF=FLandscapeXYOffsetVertexFactory, Platform=PCD3D_SM5
ShouldCache: Mat=1, VF=1, Shader=0
MaterialUsageDesc: LightingModel=MSM_DefaultLit, BlendMode=BLEND_Opaque, SpecialEngine=0, TwoSided=0, TSNormal=1, InjectEmissiveIntoLPV=0, Masked=0, Distorted=0, Usage=

{bUsedWithStaticLighting,bUsedWithLandscape}

KERNELBASE.dll

{0x00007ff93f838b9c}

+ 0 bytes
UE4Editor-Core.dll!FOutputDeviceWindowsError::Serialize()

{0x00007ff91a015a3f}

+ 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Core.dll!FOutputDevice::Logf__VA()

{0x00007ff919ecf1e8}

+ 159 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Core.dll!FDebug::AssertFailed()

{0x00007ff919ea03c2}

+ 65 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Engine.dll!FMaterial::GetShader()

{0x00007ff903ce0caa}

+ 571 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Landscape.dll!FLandscapeGrassWeightExporter_RenderThread::RenderLandscapeComponentToTexture_RenderThread()

{0x00007ff908d246ba}

+ 101 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Landscape.dll!TGraphTask<`FLandscapeGrassWeightExporter::FLandscapeGrassWeightExporter'::`18'::EURCMacro_FDrawSceneCommand>::ExecuteTask()

{0x00007ff908ce4a41}

+ 18 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Core.dll!FTaskThread::ProcessTasks()

{0x00007ff919d55e2e}

+ 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-Core.dll!FTaskThread::ProcessTasksUntilQuit()

{0x00007ff919d5605d}

+ 0 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.8\engine\sou
UE4Editor-RenderCore.dll!