Fix several invalid connection closure issues.

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -32,7 +32,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
         if (!connection.getPlayer().isActive()) {
             // Connection was left open accidentally. Close it so as to avoid "You logged in from another location"
             // errors.
-            connection.getMinecraftConnection().close();
+            connection.disconnect();
             return;
         }
 
@@ -99,7 +99,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
         if (!connection.getPlayer().isActive()) {
             // Connection was left open accidentally. Close it so as to avoid "You logged in from another location"
             // errors.
-            connection.getMinecraftConnection().close();
+            connection.disconnect();
             return;
         }
 

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/VelocityServerConnection.java

@@ -149,9 +149,11 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     }
 
     public void disconnect() {
+        if (minecraftConnection != null) {
             minecraftConnection.close();
             minecraftConnection = null;
         }
+    }
 
     @Override
     public String toString() {