[unity] Added missing null checks in SkeletonRendererCustomMaterials. Closes #2081.

spine-unity/Assets/Spine/Runtime/spine-unity/Components/SkeletonRendererCustomMaterials/SkeletonRendererCustomMaterials.cs

@@ -90,7 +90,8 @@ namespace Spine.Unity {
 					continue;
 
 				Slot slotObject = skeletonRenderer.skeleton.FindSlot(slotMaterialOverride.slotName);
-				skeletonRenderer.CustomSlotMaterials[slotObject] = slotMaterialOverride.material;
+				if (slotObject != null)
+					skeletonRenderer.CustomSlotMaterials[slotObject] = slotMaterialOverride.material;
 			}
 		}
 
@@ -106,7 +107,8 @@ namespace Spine.Unity {
 					continue;
 
 				Slot slotObject = skeletonRenderer.skeleton.FindSlot(slotMaterialOverride.slotName);
-
+				if (slotObject == null)
+					continue;
 				Material currentMaterial;
 				if (!skeletonRenderer.CustomSlotMaterials.TryGetValue(slotObject, out currentMaterial))
 					continue;