Exception printing HiveWall39671 at (124, 0, 119): System.NullReferenceException

[Alude]

when i try to build my custom wall i get this error

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Exception printing HiveWall39671 at (124, 0, 119): System.NullReferenceException: Object reference not set to an instance of an object
  at Verse.Graphic_Appearances.MatSingleFor (Verse.Thing thing) [0x00007] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
  at Verse.Graphic_Linked.LinkedDrawMatFrom (Verse.Thing parent, Verse.IntVec3 cell) [0x00039] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
  at Verse.Graphic_Linked.Print (Verse.SectionLayer layer, Verse.Thing thing) [0x00008] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
  at Verse.Graphic_LinkedCornerFiller.Print (Verse.SectionLayer layer, Verse.Thing thing) [0x00000] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
  at Verse.Thing.Print (Verse.SectionLayer layer) [0x00006] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
  at Verse.ThingWithComps.Print (Verse.SectionLayer layer) [0x00000] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
  at Verse.SectionLayer_ThingsGeneral.TakePrintFrom (Verse.Thing t) [0x00000] in <0ee2c524c4be441e9b7f8bfcb20aca6f>:0
Verse.Log:Error(String, Boolean)
Verse.SectionLayer_ThingsGeneral:TakePrintFrom(Thing)
Verse.SectionLayer_Things:Regenerate()
Verse.Section:RegenerateLayers(MapMeshFlag)
Verse.MapDrawer:TryUpdateSection(Section)
Verse.MapDrawer:MapMeshDrawerUpdate_First()
Verse.Map:MapUpdate()
Verse.Game:UpdatePlay()
Verse.Root_Play:Update()
but the object is there but its invisible
my wall xml is

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<ThingDef ParentName="BuildingBase" Name="HiveWallBase">
    <defName>HiveWall</defName>
    <label>Hivewall</label>
    <description>A hivewall made from biomaterials.Very weak.</description>
    <thingClass>Building</thingClass>
    <category>Building</category>
    <graphicData>
      <texPath>Things/Building/Linked/hivewallatlas</texPath>
      <graphicClass>Graphic_Appearances</graphicClass>
      <linkType>CornerFiller</linkType>
      <linkFlags>
        <li>Wall</li>
        <li>Rock</li>
      </linkFlags>
      <damageData>
        <cornerTL>Damage/Corner</cornerTL>
        <cornerTR>Damage/Corner</cornerTR>
        <cornerBL>Damage/Corner</cornerBL>
        <cornerBR>Damage/Corner</cornerBR>
        <edgeTop>Damage/Edge</edgeTop>
        <edgeBot>Damage/Edge</edgeBot>
        <edgeLeft>Damage/Edge</edgeLeft>
        <edgeRight>Damage/Edge</edgeRight>
      </damageData>
    </graphicData>
<costList>
      <Steel>1</Steel>
    </costList>
    <uiIconPath>hivewalluiicon</uiIconPath>
    <statBases>
      <MaxHitPoints>65</MaxHitPoints>
      <WorkToBuild>135</WorkToBuild>
      <Flammability>1.0</Flammability>
      <MeditationFocusStrength>0.22</MeditationFocusStrength>
    </statBases>
    <leaveResourcesWhenKilled>false</leaveResourcesWhenKilled>
    <altitudeLayer>Building</altitudeLayer>
    <passability>Impassable</passability>
    <blockWind>true</blockWind>
    <castEdgeShadows>true</castEdgeShadows>
    <fillPercent>1</fillPercent>
    <coversFloor>true</coversFloor>
    <placingDraggableDimensions>1</placingDraggableDimensions>
    <tickerType>Never</tickerType>
    <rotatable>false</rotatable>
    <selectable>true</selectable>
    <neverMultiSelect>true</neverMultiSelect>
    <terrainAffordanceNeeded>Heavy</terrainAffordanceNeeded> <!-- Best affordance needed for stone -->
    <holdsRoof>true</holdsRoof>
    <designationCategory>Structure</designationCategory>
    <staticSunShadowHeight>1.0</staticSunShadowHeight>
    <blockLight>true</blockLight>
    <canOverlapZones>false</canOverlapZones>
    <fertility>0</fertility>
    <building>
      <isInert>true</isInert>
      <ai_chillDestination>false</ai_chillDestination>
      <blueprintGraphicData>
        <texPath>blueprintatlas</texPath>
      </blueprintGraphicData>
    </building>
    <comps>
      <li Class="CompProperties_MeditationFocus">
        <statDef>MeditationFocusStrength</statDef>
        <focusTypes>
          <li>Minimal</li>
        </focusTypes>
      </li>
    </comps>
    <damageMultipliers>
      <li>
        <damageDef>Bomb</damageDef>
        <multiplier>2</multiplier>
      </li>
    </damageMultipliers>
  </ThingDef>
how would i fix this?