Add more thread-safety checks

core/src/main/java/me/filoghost/holographicdisplays/core/api/current/DefaultHolographicDisplaysAPI.java

@@ -39,7 +39,6 @@ class DefaultHolographicDisplaysAPI implements HolographicDisplaysAPI {
     public @NotNull Hologram createHologram(@NotNull Location location) {
         Preconditions.notNull(location, "location");
         Preconditions.notNull(location.getWorld(), "location.getWorld()");
-        Preconditions.checkMainThread("async hologram creation");
 
         return apiHologramManager.createHologram(ImmutablePosition.of(location), plugin);
     }
@@ -48,7 +47,6 @@ class DefaultHolographicDisplaysAPI implements HolographicDisplaysAPI {
     public @NotNull Hologram createHologram(@NotNull Position position) {
         Preconditions.notNull(position, "position");
         Preconditions.notNull(position.getWorldName(), "position.getWorldName()");
-        Preconditions.checkMainThread("async hologram creation");
 
         return apiHologramManager.createHologram(ImmutablePosition.of(position), plugin);
     }

core/src/main/java/me/filoghost/holographicdisplays/core/api/v2/V2HologramsAPIProvider.java

@@ -38,7 +38,6 @@ public class V2HologramsAPIProvider extends HologramsAPIProvider {
         Preconditions.notNull(plugin, "plugin");
         Preconditions.notNull(source, "source");
         Preconditions.notNull(source.getWorld(), "source.getWorld()");
-        Preconditions.checkMainThread("async hologram creation");
 
         return hologramManager.createHologram(ImmutablePosition.of(source), plugin);
     }

core/src/main/java/me/filoghost/holographicdisplays/core/base/BaseHologramManager.java

@@ -5,6 +5,7 @@
  */
 package me.filoghost.holographicdisplays.core.base;
 
+import me.filoghost.fcommons.Preconditions;
 import org.bukkit.Chunk;
 import org.bukkit.World;
 
@@ -20,6 +21,8 @@ public abstract class BaseHologramManager<H extends BaseHologram> {
     private final List<H> unmodifiableHologramsView = Collections.unmodifiableList(holograms);
 
     protected void addHologram(H hologram) {
+        Preconditions.checkMainThread("async hologram create");
+
         holograms.add(hologram);
     }
 
@@ -28,11 +31,15 @@ public abstract class BaseHologramManager<H extends BaseHologram> {
     }
 
     public void deleteHologram(H hologram) {
+        Preconditions.checkMainThread("async hologram delete");
+
         hologram.setDeleted();
         holograms.remove(hologram);
     }
 
     public void deleteHologramsIf(Predicate<H> condition) {
+        Preconditions.checkMainThread("async hologram delete");
+
         Iterator<H> iterator = holograms.iterator();
         while (iterator.hasNext()) {
             H hologram = iterator.next();
@@ -44,6 +51,8 @@ public abstract class BaseHologramManager<H extends BaseHologram> {
     }
 
     public void deleteHolograms() {
+        Preconditions.checkMainThread("async hologram delete");
+
         Iterator<H> iterator = holograms.iterator();
         while (iterator.hasNext()) {
             H hologram = iterator.next();

core/src/main/java/me/filoghost/holographicdisplays/core/listener/ChunkListener.java

@@ -48,6 +48,16 @@ public class ChunkListener implements Listener {
     @EventHandler(priority = EventPriority.MONITOR, ignoreCancelled = true)
     public void onChunkUnload(ChunkUnloadEvent event) {
         Chunk chunk = event.getChunk();
+
+        // Always execute on the main thread
+        if (Bukkit.isPrimaryThread()) {
+            onChunkUnload(chunk);
+        } else {
+            Bukkit.getScheduler().runTask(plugin, () -> onChunkUnload(chunk));
+        }
+    }
+
+    private void onChunkUnload(Chunk chunk) {
         apiHologramManager.onChunkUnload(chunk);
         v2HologramManager.onChunkUnload(chunk);
     }
@@ -61,7 +71,7 @@ public class ChunkListener implements Listener {
             return;
         }
 
-        // In case another plugin loads the chunk asynchronously, always make sure to load the holograms on the main thread
+        // Always execute on the main thread
         if (Bukkit.isPrimaryThread()) {
             onChunkLoad(chunk);
         } else {