FlxSubState: fix close() if used in two different states (#1971)

If you have persistent substates which you open from multiple parent states, you want the parent state of the substate to change even if the state has already been created, otherwise _parentState will be incorrect when you open the substate from a second location.

flixel/FlxState.hx

@@ -112,10 +112,11 @@ class FlxState extends FlxGroup
 				FlxG.inputs.onStateSwitch();
 			}
 
+			subState._parentState = this;
+
 			if (!subState._created)
 			{
 				subState._created = true;
-				subState._parentState = this;
 				subState.create();
 			}
 		}

tests/unit/src/flixel/FlxSubStateTest.hx

@@ -37,4 +37,31 @@ class FlxSubStateTest extends FlxTest
 
 		Assert.areEqual(subState2, FlxG.state.subState);
 	}
+
+	@Test // #1971
+	function testOpenPersistentSubStateFromNewParent()
+	{
+		var state1 = new FlxState();
+		var state2 = new FlxState();
+		state1.destroySubStates = false;
+		FlxG.switchState(state1);
+		step();
+		FlxG.state.openSubState(subState1);
+		step();
+
+		Assert.areEqual(state1.subState, subState1);
+		subState1.close();
+		step();
+		Assert.isNull(state1.subState);
+
+		FlxG.switchState(state2);
+		step();
+		FlxG.state.openSubState(subState1);
+		step();
+
+		Assert.areEqual(state2.subState, subState1);
+		subState1.close();
+		step();
+		Assert.isNull(state2.subState);
+	}
 }