safety = null

- Do not crash when instances passed to api methods are null.
  - Ignore null crumbling instances.
  - Do not attempt to steal null instances.
- Add nullability annotations where relevant.
- Update docs to indicate null behavior.

src/main/java/com/jozufozu/flywheel/api/backend/Engine.java

@@ -2,6 +2,8 @@ package com.jozufozu.flywheel.api.backend;
 
 import java.util.List;
 
+import org.jetbrains.annotations.NotNull;
+
 import com.jozufozu.flywheel.api.event.RenderContext;
 import com.jozufozu.flywheel.api.event.RenderStage;
 import com.jozufozu.flywheel.api.instance.Instance;
@@ -65,6 +67,6 @@ public interface Engine extends InstancerProvider {
 	 * @param pos The position of the block.
 	 * @param instances The instances associated with the BE at this position.
 	 */
-	record CrumblingBlock(int progress, BlockPos pos, List<Instance> instances) {
+	record CrumblingBlock(int progress, BlockPos pos, List<@NotNull Instance> instances) {
 	}
 }

src/main/java/com/jozufozu/flywheel/api/instance/Instancer.java

@@ -1,5 +1,7 @@
 package com.jozufozu.flywheel.api.instance;
 
+import org.jetbrains.annotations.Nullable;
+
 /**
  * An instancer is how you interact with an instanced model.
  * <p>
@@ -31,11 +33,13 @@ public interface Instancer<I extends Instance> {
 	 * <br><br>
 	 * If the given instance is already owned by this instancer, this method does nothing.
 	 * <br>
+	 * If the given instance is {@code null}, this method does nothing.
+	 * <br>
 	 * If the given instance was created by a different backend, the behavior of this method is undefined.
 	 *
 	 * @param instance The instance to steal.
 	 */
-	void stealInstance(I instance);
+	void stealInstance(@Nullable I instance);
 
 	/**
 	 * Populate arr with new instances of this model.

src/main/java/com/jozufozu/flywheel/api/visual/BlockEntityVisual.java

@@ -2,6 +2,8 @@ package com.jozufozu.flywheel.api.visual;
 
 import java.util.function.Consumer;
 
+import org.jetbrains.annotations.Nullable;
+
 import com.jozufozu.flywheel.api.instance.Instance;
 
 import net.minecraft.world.level.block.entity.BlockEntity;
@@ -10,8 +12,10 @@ public interface BlockEntityVisual<T extends BlockEntity> extends Visual {
 	/**
 	 * Collect all instances that should render with a crumbling overlay
 	 * when the block corresponding to this visual is being broken.
+	 * <br>
+	 * Passing {@code null} to the consumer has no effect.
 	 *
 	 * @param consumer A consumer to provide instances to.
 	 */
-	void collectCrumblingInstances(Consumer<Instance> consumer);
+	void collectCrumblingInstances(Consumer<@Nullable Instance> consumer);
 }

src/main/java/com/jozufozu/flywheel/backend/engine/AbstractInstancer.java

@@ -36,6 +36,10 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 
 	@Override
 	public void stealInstance(I instance) {
+		if (instance == null) {
+			return;
+		}
+
 		var instanceHandle = instance.handle();
 
 		if (!(instanceHandle instanceof InstanceHandleImpl handle)) {

src/main/java/com/jozufozu/flywheel/impl/visualization/VisualizationManagerImpl.java

@@ -230,7 +230,11 @@ public class VisualizationManagerImpl implements VisualizationManager {
 
 			List<Instance> instances = new ArrayList<>();
 
-			visual.collectCrumblingInstances(instances::add);
+			visual.collectCrumblingInstances(instance -> {
+				if (instance != null) {
+					instances.add(instance);
+				}
+			});
 
 			if (instances.isEmpty()) {
 				// The visual doesn't want to render anything crumbling.