fix null pointer error

Assets/Scripts/Game/Ball.cs

@@ -4,7 +4,7 @@ using UnityEngine;
 namespace Game {
 	public class Ball : NetworkBehaviour {
 
-		private float SpeedGain { get; } = GameManager.BalanceValues.ballSpeedMultiplier;
+		private float SpeedGain { get; set; }
 
 		public Rigidbody2D Rb { get; private set; }
 
@@ -22,6 +22,7 @@ namespace Game {
 		private void Awake() {
 			Rb = GetComponent<Rigidbody2D>();
 			Collider = GetComponent<CircleCollider2D>();
+			SpeedGain = GameManager.BalanceValues.ballSpeedMultiplier;
 		}
 
 		private void OnCollisionEnter2D(Collision2D other) {