Fixed null pointer exception after changing frameskip

app/src/main/java/de/hems/trafficsim/MainActivity.java

@@ -175,15 +175,25 @@ public class MainActivity extends AppCompatActivity implements SeekBar.OnSeekBar
 
         float newMaxVelocity = ((SeekBar) findViewById(R.id.maxVelocitySeekBar)).getProgress();
         SeekBar brakeProbabilitySeekBar = (SeekBar) findViewById(R.id.brakeProbabilitySeekBar);
-        float newBrakeProb = (float)brakeProbabilitySeekBar.getProgress() / (float)brakeProbabilitySeekBar.getMax();
+        float newBrakeProb = (float)brakeProbabilitySeekBar.getProgress()
+                / (float)brakeProbabilitySeekBar.getMax();
 
         int newDelay = ((SeekBar)(findViewById(R.id.simDelaySeekBar))).getProgress();
         int newFrameskip = ((SeekBar)(findViewById(R.id.frameskipSeekBar))).getProgress();
 
-        this.track = new Track(newNoOfVehicles, newTrackLength, newBrakeProb, newMaxVelocity, newDelay, defaultHistoryLength);
+        boolean wasRunning = this.worker != null;
+        if (wasRunning) {
+            this.worker.setStop(true);
+            try {
+                this.worker.join();
+            } catch (InterruptedException ex) { }
+        }
+
+        this.track = new Track(newNoOfVehicles, newTrackLength, newBrakeProb, newMaxVelocity,
+                newDelay, defaultHistoryLength);
         this.renderer.setTrack(this.track);
-        if (this.worker != null) { // There was a simulation running already
-            this.stopWorker();
+
+        if (wasRunning) {
             this.worker = new Worker(track, this, renderer, newFrameskip);
             this.worker.start();
         }
@@ -211,7 +221,9 @@ public class MainActivity extends AppCompatActivity implements SeekBar.OnSeekBar
         } else if (seekBar == (SeekBar)(findViewById(R.id.frameskipSeekBar))) {
             TextView tv = (TextView)(findViewById(R.id.frameSkipTextView));
             tv.setText(String.valueOf(seekBar.getProgress()));
-            this.worker.setFrameskip(seekBar.getProgress());
+            if (this.worker != null) {
+                this.worker.setFrameskip(seekBar.getProgress());
+            }
         }
     }