Allow decal projectors with null materials, so that users can still modify projector decals even when HDRP is not set as current pipeline.

com.unity.render-pipelines.high-definition/HDRP/Decal/DecalProjectorComponent.cs

             }
 
             if (m_Handle != null)
+            { 
+                m_Handle = null;
+            }
+
             Vector4 uvScaleBias = new Vector4(m_UVScale.x, m_UVScale.y, m_UVBias.x, m_UVBias.y);
             m_Handle = DecalSystem.instance.AddDecal(transform, m_DrawDistance, m_FadeScale, uvScaleBias, m_Material);
         }
-            DecalSystem.instance.RemoveDecal(m_Handle);
-            m_Handle = null;
+            if (m_Handle != null)
+            { 
+                DecalSystem.instance.RemoveDecal(m_Handle);
+                m_Handle = null;
+            }
         }
 
         // Declare the method signature of the delegate to call.

com.unity.render-pipelines.high-definition/HDRP/Decal/DecalSystem.cs

     public class DecalSystem
     {
         public const int kInvalidIndex = -1;
+        public const int kNullMaterialIndex = int.MaxValue;
         public class DecalHandle
         {
             public DecalHandle(int index, int materialID)
         {
             public void InitializeMaterialValues()
             {
+                if (m_Material == null)
+                    return;
                 m_Diffuse.m_Texture = m_Material.GetTexture("_BaseColorMap");
                 m_Normal.m_Texture = m_Material.GetTexture("_NormalMap");
                 m_Mask.m_Texture = m_Material.GetTexture("_MaskMap");
             public DecalSet(Material material)
             {
-                m_Material = material;
+                m_Material = material;              
                 InitializeMaterialValues();
             }
 
 
             public void UpdateCachedData(Transform transform, float drawDistance, float fadeScale, Vector4 uvScaleBias, DecalHandle handle)
             {
+                if (m_Material == null)
+                    return;
                 int index = handle.m_Index;
                 m_CachedDecalToWorld[index] = transform.localToWorldMatrix;
 
 
             public void BeginCull()
             {
+                if (m_Material == null)
+                    return;
                 if (m_CullingGroup != null)
                 {
                     Debug.LogError("Begin/EndCull() called out of sequence for decal projectors.");
 
             public int QueryCullResults()
             {
+                if (m_Material == null)
+                    return 0;
                 m_NumResults = m_CullingGroup.QueryIndices(true, m_ResultIndices, 0);
                 return m_NumResults;
             }
 
             public void CreateDrawData()
             {
+                if (m_Material == null)
+                    return;
                 if (m_NumResults == 0)
                     return;
                 // only add if anything in this decal set is visible.
 
             public void EndCull()
             {
+                if (m_Material == null)
+                    return;
                 if (m_CullingGroup == null)
                 {
                     Debug.LogError("Begin/EndCull() called out of sequence for decal projectors.");
 
             public void RenderIntoDBuffer(CommandBuffer cmd)
             {
+                if (m_Material == null)
+                    return;
                 if (m_NumResults == 0)
                     return;
                 int batchIndex = 0;
         public DecalHandle AddDecal(Transform transform, float drawDistance, float fadeScale, Vector4 uvScaleBias, Material material)
         {
             DecalSet decalSet = null;
-            int key = material.GetInstanceID();
+            int key = material != null ? material.GetInstanceID() : kNullMaterialIndex;
             if (!m_DecalSets.TryGetValue(key, out decalSet))
             {
                 decalSet = new DecalSet(material);