fix: processorExtent can be null when setting masks too

- fixes #2379

worldedit-core/src/main/java/com/sk89q/worldedit/EditSession.java

@@ -631,10 +631,12 @@ public class EditSession extends PassthroughExtent implements AutoCloseable {
         }
         MaskingExtent maskingExtent = new ExtentTraverser<>(getExtent()).findAndGet(MaskingExtent.class);
         if (maskingExtent == null && mask != Masks.alwaysTrue()) {
-            ExtentTraverser<ExtentBatchProcessorHolder> processorExtent =
-                    new ExtentTraverser<>(getExtent()).find(ExtentBatchProcessorHolder.class);
-            maskingExtent =
-                    new ProcessorTraverser<>(processorExtent.get().getProcessor()).find(MaskingExtent.class);
+            ExtentBatchProcessorHolder processorExtent =
+                    new ExtentTraverser<>(getExtent()).findAndGet(ExtentBatchProcessorHolder.class);
+            if (processorExtent != null) {
+                maskingExtent =
+                        new ProcessorTraverser<>(processorExtent.getProcessor()).find(MaskingExtent.class);
+            }
         }
         if (maskingExtent != null) {
             Mask oldMask = maskingExtent.getMask();