Fix #20: skip renderers that do not have a sharedMaterial

…nderers, apparently)

Author: JonnyOThan

Source/MaterialReplacement.cs

@@ -46,6 +46,7 @@ public void ApplyToSharedMaterialIfNotIgnored(Renderer renderer)
 		{
 			if (MatchIgnored(renderer)) return;
 			var sharedMat = renderer.sharedMaterial;
+			if (sharedMat == null) return;
 			if (!replacedMaterials.TryGetValue(sharedMat, out var replacementMat)) {
 				replacementMat = materialDef.Instantiate(sharedMat);
 				replacedMaterials[sharedMat] = replacementMat;