#937: Fixes related to unplaced BlockStates and the recent world generation changes.

* CraftBlockState#getWorldHandle() would previously run into a NPE when being invoked for a non-placed BlockState. It now returns null in this case.
* CraftChest#getInventory() would previously encounter this NPE when being called for a non-placed BlockState. It will now automatically forward the call to #getBlockInventory() when it is being called for either a non-placed BlockState, or during world generation.
* CraftStructureBlock#applyTo was able to run into a NPE when being called for non-placed BlockStates (this is for example called by #getSnapshotNBT()).
* Replaced all no-world-generation preconditions with a general 'ensureNoWorldGeneration' method.
* Removed a few superfluous #isPlaced() calls. If getWorldHandle() reutrns a World, this already implies that the BlockState is placed.

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

@@ -23,7 +23,7 @@ public class CraftBeacon extends CraftBlockEntityState<TileEntityBeacon> impleme
 
     @Override
     public Collection<LivingEntity> getEntitiesInRange() {
-        Preconditions.checkState(getWorldHandle() instanceof net.minecraft.world.level.World, "Can't get entities during world generation");
+        ensureNoWorldGeneration();
 
         TileEntity tileEntity = this.getTileEntityFromWorld();
         if (tileEntity instanceof TileEntityBeacon) {

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

@@ -60,7 +60,7 @@ public class CraftBeehive extends CraftBlockEntityState<TileEntityBeehive> imple
 
     @Override
     public List<Bee> releaseEntities() {
-        Preconditions.checkState(getWorldHandle() instanceof net.minecraft.world.level.World, "Can't release entities during world generation");
+        ensureNoWorldGeneration();
 
         List<Bee> bees = new ArrayList<>();
 

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

@@ -57,20 +57,33 @@ public class CraftBlockState implements BlockState {
         }
     }
 
+    // Returns null if weakWorld is not available and the BlockState is not placed.
+    // If this returns a World instead of only a GeneratorAccess, this implies that this BlockState is placed.
     public GeneratorAccess getWorldHandle() {
         if (weakWorld == null) {
-            return world.getHandle();
+            return this.isPlaced() ? world.getHandle() : null;
         }
 
         GeneratorAccess access = weakWorld.get();
         if (access == null) {
             weakWorld = null;
-            return world.getHandle();
+            return this.isPlaced() ? world.getHandle() : null;
         }
 
         return access;
     }
 
+    protected final boolean isWorldGeneration() {
+        GeneratorAccess generatorAccess = this.getWorldHandle();
+        return generatorAccess != null && !(generatorAccess instanceof World);
+    }
+
+    protected final void ensureNoWorldGeneration() {
+        if (isWorldGeneration()) {
+            throw new IllegalStateException("This operation is not supported during world generation!");
+        }
+    }
+
     @Override
     public World getWorld() {
         requirePlaced();

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

@@ -37,10 +37,8 @@ public class CraftChest extends CraftLootable<TileEntityChest> implements Chest
 
     @Override
     public Inventory getInventory() {
-        Preconditions.checkState(getWorldHandle() instanceof net.minecraft.world.level.World, "Can't get inventory during world generation, use getBlockInventory() instead");
-
         CraftInventory inventory = (CraftInventory) this.getBlockInventory();
-        if (!isPlaced()) {
+        if (!isPlaced() || isWorldGeneration()) {
             return inventory;
         }
 

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

@@ -47,10 +47,8 @@ public class CraftDispenser extends CraftLootable<TileEntityDispenser> implement
 
     @Override
     public boolean dispense() {
-        Preconditions.checkState(getWorldHandle() instanceof net.minecraft.world.level.World, "Can't dispense during world generation");
+        ensureNoWorldGeneration();
-
         Block block = getBlock();
-
         if (block.getType() == Material.DISPENSER) {
             CraftWorld world = (CraftWorld) this.getWorld();
             BlockDispenser dispense = (BlockDispenser) Blocks.DISPENSER;

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

@@ -34,10 +34,8 @@ public class CraftDropper extends CraftLootable<TileEntityDropper> implements Dr
 
     @Override
     public void drop() {
-        Preconditions.checkState(getWorldHandle() instanceof net.minecraft.world.level.World, "Can't drop during world generation");
+        ensureNoWorldGeneration();
-
         Block block = getBlock();
-
         if (block.getType() == Material.DROPPER) {
             CraftWorld world = (CraftWorld) this.getWorld();
             BlockDropper drop = (BlockDropper) Blocks.DROPPER;

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

@@ -81,9 +81,8 @@ public class CraftJukebox extends CraftBlockEntityState<TileEntityJukeBox> imple
 
     @Override
     public boolean eject() {
-        Preconditions.checkState(getWorldHandle() instanceof net.minecraft.world.level.World, "Can't eject during world generation");
+        ensureNoWorldGeneration();
 
-        requirePlaced();
         TileEntity tileEntity = this.getTileEntityFromWorld();
         if (!(tileEntity instanceof TileEntityJukeBox)) return false;
 

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

@@ -42,7 +42,7 @@ public class CraftLectern extends CraftBlockEntityState<TileEntityLectern> imple
     public boolean update(boolean force, boolean applyPhysics) {
         boolean result = super.update(force, applyPhysics);
 
-        if (result && this.isPlaced() && this.getType() == Material.LECTERN && getWorldHandle() instanceof net.minecraft.world.level.World) {
+        if (result && this.getType() == Material.LECTERN && getWorldHandle() instanceof net.minecraft.world.level.World) {
             BlockLectern.a(this.world.getHandle(), this.getPosition(), this.getHandle());
         }
 

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

@@ -180,7 +180,7 @@ public class CraftStructureBlock extends CraftBlockEntityState<TileEntityStructu
         // Ensure block type is correct
         if (access instanceof net.minecraft.world.level.World) {
             tileEntity.setUsageMode(tileEntity.getUsageMode());
-        } else {
+        } else if (access != null) {
             // Custom handle during world generation
             // From TileEntityStructure#setUsageMode(BlockPropertyStructureMode)
             net.minecraft.world.level.block.state.IBlockData data = access.getType(this.getPosition());