First fix REntity for 1.21

SpigotCore/SpigotCore_21/src/de/steamwar/core/BountifulWrapper21.java

@@ -33,7 +33,7 @@ public class BountifulWrapper21 extends BountifulWrapper9 {
             return (packet, x, y, z, pitch, yaw) -> {
                 PositionMoveRotation pos = field.get(packet);
 
-                field.set(packet, new PositionMoveRotation(new Vec3D(x, y, z), pos.b(), yaw, pitch));
+                field.set(packet, new PositionMoveRotation(new Vec3D(x, y, z), pos == null ? new Vec3D(0, 0, 0) : pos.b(), yaw, pitch));
             };
         } catch (IllegalArgumentException e) {
             return super.getPositionSetter(packetClass, fieldOffset);

SpigotCore/SpigotCore_Main/src/de/steamwar/Reflection.java

@@ -177,9 +177,30 @@ public final class Reflection {
 		}
 
 		public void set(Object target, Object value) {
+			// This now works for Fields in records!
 			try {
-				f.set(target, value);
-			} catch (IllegalAccessException e) {
+				long offset = Unsafe.getUnsafe().objectFieldOffset(f);
+				Class<?> type = f.getType();
+				if (type == int.class) {
+					Unsafe.getUnsafe().putInt(target, offset, (Integer) value);
+				} else if (type == float.class) {
+					Unsafe.getUnsafe().putFloat(target, offset, (Float) value);
+				} else if (type == double.class) {
+					Unsafe.getUnsafe().putDouble(target, offset, (Double) value);
+				} else if (type == boolean.class) {
+					Unsafe.getUnsafe().putBoolean(target, offset, (Boolean) value);
+				} else if (type == byte.class) {
+					Unsafe.getUnsafe().putByte(target, offset, (Byte) value);
+				} else if (type == char.class) {
+					Unsafe.getUnsafe().putChar(target, offset, (Character) value);
+				} else if (type == short.class) {
+					Unsafe.getUnsafe().putShort(target, offset, (Short) value);
+				} else if (type == long.class) {
+					Unsafe.getUnsafe().putLong(target, offset, (Long) value);
+				} else {
+					Unsafe.getUnsafe().putReference(target, offset, value);
+				}
+			} catch (Exception e) {
 				throw new IllegalArgumentException("Cannot write field", e);
 			}
 		}

SpigotCore/SpigotCore_Main/src/de/steamwar/entity/REntity.java

@@ -396,10 +396,26 @@ public class REntity {
     public static final Class<?> teleportPacket = Reflection.getClass("net.minecraft.network.protocol.game.ClientboundTeleportEntityPacket");
     public static final Reflection.Field<Integer> teleportEntity = Reflection.getField(teleportPacket, int.class, 0);
     public static final BountifulWrapper.PositionSetter teleportPosition = BountifulWrapper.impl.getPositionSetter(teleportPacket, Core.getVersion() == 8 ? 1 : 0);
+    public static final Class<?> relative;
+    public static final Reflection.Field<Set> relatives;
+    static {
+        if (Core.getVersion() >= 21) {
+            relative = Reflection.getClass("net.minecraft.world.entity.Relative");
+            relatives = Reflection.getField(teleportPacket, Set.class, 0);
+        } else {
+            relative = null;
+            relatives = null;
+        }
+    }
     private Object getTeleportPacket(){
         Object packet = Reflection.newInstance(teleportPacket);
         teleportEntity.set(packet, entityId);
         teleportPosition.set(packet, x, y, z, pitch, yaw);
+        if (Core.getVersion() >= 21) {
+            // https://mappings.dev/1.21.3/net/minecraft/world/entity/Relative.html
+            Object[] constants = relative.getEnumConstants();
+            relatives.set(packet, new HashSet<>(Arrays.asList(constants[0], constants[1], constants[2], constants[3], constants[4], constants[5])));
+        }
         return packet;
     }