SPIGOT-5650: Lectern.setPage(int) causes a NullPointerException

nms-patches/TileEntityLectern.patch

@@ -89,6 +89,15 @@
          }
  
          @Override
+@@ -135,7 +197,7 @@
+         if (j != this.page) {
+             this.page = j;
+             this.update();
+-            BlockLectern.a(this.getWorld(), this.getPosition(), this.getBlock());
++            if (this.world != null) BlockLectern.a(this.getWorld(), this.getPosition(), this.getBlock()); // CraftBukkit
+         }
+ 
+     }
 @@ -158,6 +220,32 @@
          return itemstack;
      }

src/main/java/org/bukkit/craftbukkit/block/CraftLectern.java

@@ -1,5 +1,6 @@
 package org.bukkit.craftbukkit.block;
 
+import net.minecraft.server.BlockLectern;
 import net.minecraft.server.TileEntityLectern;
 import org.bukkit.Material;
 import org.bukkit.block.Block;
@@ -40,4 +41,15 @@ public class CraftLectern extends CraftBlockEntityState<TileEntityLectern> imple
 
         return new CraftInventoryLectern(this.getTileEntity().inventory);
     }
+
+    @Override
+    public boolean update(boolean force, boolean applyPhysics) {
+        boolean result = super.update(force, applyPhysics);
+
+        if (result && this.isPlaced() && this.getType() == Material.LECTERN) {
+            BlockLectern.a(this.world.getHandle(), this.getPosition(), this.getHandle());
+        }
+
+        return result;
+    }
 }