Added preconditions to BreadthFirstSearch and subclasses.

src/main/java/com/sk89q/worldedit/function/visitor/BreadthFirstSearch.java

@@ -27,6 +27,8 @@ import com.sk89q.worldedit.function.RegionFunction;
 
 import java.util.*;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * Performs a breadth-first search starting from points added with
  * {@link #visit(com.sk89q.worldedit.Vector)}. The search continues
@@ -51,7 +53,8 @@ public abstract class BreadthFirstSearch implements Operation {
      *
      * @param function the function to apply to visited blocks
      */
-    public BreadthFirstSearch(RegionFunction function) {
+    protected BreadthFirstSearch(RegionFunction function) {
+        checkNotNull(function);
         this.function = function;
         addAxes();
     }

src/main/java/com/sk89q/worldedit/function/visitor/DownwardVisitor.java

@@ -25,6 +25,8 @@ import com.sk89q.worldedit.function.mask.Mask;
 
 import java.util.List;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * Visits adjacent points on the same X-Z plane as long as the points
  * pass the given mask, and then executes the provided region
@@ -45,6 +47,7 @@ public class DownwardVisitor extends RecursiveVisitor {
      */
     public DownwardVisitor(Mask mask, RegionFunction function, int baseY) {
         super(mask, function);
+        checkNotNull(mask);
 
         this.baseY = baseY;
 

src/main/java/com/sk89q/worldedit/function/visitor/LayerVisitor.java

@@ -19,7 +19,6 @@
 
 package com.sk89q.worldedit.function.visitor;
 
-import com.sk89q.worldedit.EditSession;
 import com.sk89q.worldedit.Vector;
 import com.sk89q.worldedit.Vector2D;
 import com.sk89q.worldedit.WorldEditException;

src/main/java/com/sk89q/worldedit/function/visitor/RecursiveVisitor.java

@@ -23,6 +23,8 @@ import com.sk89q.worldedit.Vector;
 import com.sk89q.worldedit.function.RegionFunction;
 import com.sk89q.worldedit.function.mask.Mask;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * An implementation of an {@link BreadthFirstSearch} that uses a mask to
  * determine where a block should be visited.
@@ -31,8 +33,15 @@ public class RecursiveVisitor extends BreadthFirstSearch {
 
     private final Mask mask;
 
+    /**
+     * Create a new recursive visitor.
+     *
+     * @param mask the mask
+     * @param function the function
+     */
     public RecursiveVisitor(Mask mask, RegionFunction function) {
         super(function);
+        checkNotNull(mask);
         this.mask = mask;
     }