fixed next turn button by updating it on main thread

core/src/com/unciv/ui/worldscreen/WorldScreen.kt

@@ -268,17 +268,25 @@ class WorldScreen : CameraStageBaseScreen() {
                     // So what we do is we clone all the game data and serialize the clone.
                     if(gameInfo.turns % game.settings.turnsBetweenAutosaves == 0)
                         GameSaver().saveGame(gameInfoClone, "Autosave")
+
+                    // do this on main thread
+                    Gdx.app.postRunnable {
                         nextTurnButton.enable() // only enable the user to next turn once we've saved the current one
                         updateNextTurnButton()
                     }
 
+                }
+
                 // If we put this BEFORE the save game, then we try to save the game...
                 // but the main thread does other stuff, including showing tutorials which guess what? Changes the game data
                 // BOOM! Exception!
                 // That's why this needs to be after the game is saved.
                 shouldUpdate=true
 
+                // do this on main thread
+                Gdx.app.postRunnable {
                     updateNextTurnButton()
+                }
                 Gdx.input.inputProcessor = stage
             }
         }