More crash fixes

core/src/io/anuke/mindustry/ai/Pathfinder.java

@@ -50,8 +50,11 @@ public class Pathfinder implements Runnable{
                 }
             }
 
-            //special preset which may help speed things up; this is optional
-            preloadPath(waveTeam, PathTarget.enemyCores);
+            //run next frame to try and prevent a crash
+            Core.app.post(() -> {
+                //special preset which may help speed things up; this is optional
+                preloadPath(waveTeam, PathTarget.enemyCores);
+            });
 
             start();
         });

core/src/io/anuke/mindustry/core/UI.java

@@ -178,7 +178,7 @@ public class UI implements ApplicationListener, Loadable{
 
     @Override
     public void update(){
-        if(disableUI) return;
+        if(disableUI || Core.scene == null) return;
 
         Core.scene.act();
         Core.scene.draw();