Fix crash playing a music disc from drive on server.

#48

src/main/java/dan200/computercraft/fabric/mixin/MusicDiscItemAccessor.java

@@ -0,0 +1,12 @@
+package dan200.computercraft.fabric.mixin;
+
+import net.minecraft.item.MusicDiscItem;
+import net.minecraft.sound.SoundEvent;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+@Mixin(MusicDiscItem.class)
+public interface MusicDiscItemAccessor {
+    @Accessor
+    SoundEvent getSound();
+}

src/main/java/dan200/computercraft/shared/media/items/RecordMedia.java

@@ -10,6 +10,7 @@ import javax.annotation.Nonnull;
 
 import dan200.computercraft.api.media.IMedia;
 
+import dan200.computercraft.fabric.mixin.MusicDiscItemAccessor;
 import net.minecraft.item.Item;
 import net.minecraft.item.ItemStack;
 import net.minecraft.item.MusicDiscItem;
@@ -42,6 +43,10 @@ public final class RecordMedia implements IMedia {
 
     @Override
     public SoundEvent getAudio(@Nonnull ItemStack stack) {
-        return ((MusicDiscItem) stack.getItem()).getSound();
+        Item item = stack.getItem();
+        if (!(item instanceof MusicDiscItem)) {
+            return null;
+        }
+        return ((MusicDiscItemAccessor) item).getSound();
     }
 }

src/main/resources/computercraft.mixins.json

@@ -11,7 +11,8 @@
         "SignBlockEntityAccess",
         "SoundEventAccess",
         "WorldSavePathAccess",
-        "MixinServerPlayerInteractionManager"
+        "MixinServerPlayerInteractionManager",
+        "MusicDiscItemAccessor"
     ],
     "client": [
         "AffineTransformationAccess",