Fix NPE when //genbiome expression was false.

Fixes WORLDEDIT-3467.

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

@@ -2301,7 +2301,7 @@ public class EditSession implements Extent {
 
                 try {
                     if (expression.evaluate(scaled.getX(), scaled.getZ()) <= 0) {
-                        return null;
+                        return null; // TODO should return OUTSIDE? seems to cause issues otherwise, workedaround for now
                     }
 
                     // TODO: Allow biome setting via a script variable (needs BiomeType<->int mapping)

worldedit-core/src/main/java/com/sk89q/worldedit/command/GenerationCommands.java

@@ -316,12 +316,12 @@ public class GenerationCommands {
 
     @Command(
         aliases = { "/generatebiome", "/genbiome", "/gb" },
-        usage = "<block> <expression>",
+        usage = "<biome> <expression>",
         desc = "Sets biome according to a formula.",
         help =
             "Generates a shape according to a formula that is expected to\n" +
             "return positive numbers (true) if the point is inside the shape\n" +
-            "Optionally set type/data to the desired block.\n" +
+            "Sets the biome of blocks in that shape.\n" +
             "Flags:\n" +
             "  -h to generate a hollow shape\n" +
             "  -r to use raw minecraft coordinates\n" +

worldedit-core/src/main/java/com/sk89q/worldedit/regions/shape/ArbitraryBiomeShape.java

@@ -136,7 +136,7 @@ public abstract class ArbitraryBiomeShape {
 
             if (!hollow) {
                 final BaseBiome material = getBiome(x, z, baseBiome);
-                if (material != OUTSIDE) {
+                if (material != null && material != OUTSIDE) {
                     editSession.getWorld().setBiome(position, material);
                     ++affected;
                 }