Return null if the material is null.

worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/BukkitBlockRegistry.java

@@ -38,8 +38,11 @@ public class BukkitBlockRegistry extends BundledBlockRegistry {
     @Nullable
     @Override
     public BlockMaterial getMaterial(BlockType blockType) {
-        return materialMap.computeIfAbsent(BukkitAdapter.adapt(blockType),
+        Material mat = BukkitAdapter.adapt(blockType);
-                material -> new BukkitBlockMaterial(BukkitBlockRegistry.super.getMaterial(blockType), material));
+        if (mat == null) {
+            return null;
+        }
+        return materialMap.computeIfAbsent(mat, material -> new BukkitBlockMaterial(BukkitBlockRegistry.super.getMaterial(blockType), material));
     }
 
     @Nullable