Error to start new Activity with Intent after click Button in AlertDialog

I try to start Place.Class which show more Detail of each place.

Following here I Have AboutMap extends MapActivity. I aslo create AlertDialog with Button See more detail here. But it's not work..When i run program, it crashes after Click See more detail Button. And i didn't get the exactly the result that it should be.. I think it must be wrong somewhere.. may be here

Intent i = new Intent(mContext, Place.class); //But i don't know how to fix it

Do you guys have any idea? Thanks you so much for your help

PlaceItemizedOverlay

    public class PlaceItemizedOverlay extends ItemizedOverlay<OverlayItem> {

        private Context mContext;

        private Cursor c;

        private ArrayList<OverlayItem> mOverlays = new ArrayList<OverlayItem>();

        public PlaceItemizedOverlay(Drawable defaultMarker, Context context)
        {
            super(boundCenterBottom(defaultMarker));
            mContext = context;

        }
        public PlaceItemizedOverlay(Context context)
        {
            super(boundCenterBottom(null));
            mContext = context;

        }
        public void addOverlay(OverlayItem overlay) {
            mOverlays.add(overla开发者_运维技巧y);
            populate();
        }

        @Override
        protected OverlayItem createItem(int i) {
            return mOverlays.get(i);
        }

        @Override
        public int size() {
            return mOverlays.size();    }


        @Override
        protected boolean onTap(int index) {

    //Create AlertDialog
          final OverlayItem oi = mOverlays.get(index);
          AlertDialog.Builder dialog = new AlertDialog.Builder(mContext);
          dialog.setTitle(oi.getTitle());
          dialog.setMessage(oi.getSnippet());
          dialog.setNegativeButton("Back", null);
  //Create See more detail Button
          dialog.setPositiveButton("See More Detail", new DialogInterface.OnClickListener() {

            @Override 
            public void onClick(DialogInterface dialog, int id) {
        //Line 87: Logcat tell that there are some problem around here
                    Intent i = new Intent(mContext, Place.class); 
                    i.putExtra(Constants.KEY_ID, c.getInt(
                            c.getColumnIndex(Constants.KEY_ID)));
                    i.putExtra(Constants.COL_TITLE, c.getString(
                            c.getColumnIndex(Constants.COL_TITLE)));
                    i.putExtra(Constants.COL_ADDRESS, c.getString(
                            c.getColumnIndex(Constants.COL_ADDRESS)));
                    i.putExtra(Constants.COL_CONTENT, c.getString(
                            c.getColumnIndex(Constants.COL_CONTENT)));
                    mContext.startActivity(i);
            }});
          dialog.show();
          return true;

        }


    }

AboutMap.class

public class AboutMap extends MapActivity {

    MapView mapView;
    MapController mapController;
    private static MyDB mDbHelper;
    private Cursor c;


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.aboutcm);

        mDbHelper = new MyDB(this);
        mDbHelper.createDatabase();
        mDbHelper.open();
        c = mDbHelper.getAttraction();



        mapView = (MapView) findViewById(R.id.mapview);
        mapController = mapView.getController();
        Drawable drawable = this.getResources().getDrawable(R.drawable.map_pin_3);
        List<Overlay> mapOverlays = mapView.getOverlays();
        PlaceItemizedOverlay itemizedoverlay = new PlaceItemizedOverlay(drawable, this);

        mapController.setZoom(13); 
        mapView.setBuiltInZoomControls(true);


        c.moveToFirst(); 
        do {
            String title = c.getString(c
                    .getColumnIndex(Constants.COL_TITLE));
            String address = c.getString(c
                    .getColumnIndex(Constants.COL_ADDRESS));
            int latitude = (int) (c.getDouble(c
                    .getColumnIndex(Constants.COL_LA)) * 1E6);
            int longitude = (int) (c.getDouble(c
                    .getColumnIndex(Constants.COL_LONG)) * 1E6);

            itemizedoverlay.addOverlay(new OverlayItem(new GeoPoint(latitude, longitude), title,
                    address));

            mapOverlays.add(itemizedoverlay);


        } while (c.moveToNext());


    }


    @Override
    protected boolean isRouteDisplayed() {
        // TODO Auto-generated method stub
        return false;
    }



}

Logcat

09-04 14:31:29.019: DEBUG/AndroidRuntime(975): Shutting down VM
09-04 14:31:29.029: WARN/dalvikvm(975): threadid=3: thread exiting with uncaught exception (group=0x4001b188)
09-04 14:31:29.029: ERROR/AndroidRuntime(975): Uncaught handler: thread main exiting due to uncaught exception
09-04 14:31:29.069: ERROR/AndroidRuntime(975): java.lang.NullPointerException
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at com.ctg.PlaceItemizedOverlay$1.onClick(PlaceItemizedOverlay.java:87)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:158)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at android.os.Looper.loop(Looper.java:123)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at android.app.ActivityThread.main(ActivityThread.java:4363)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at java.lang.reflect.Method.invokeNative(Native Method)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at java.lang.reflect.Method.invoke(Method.java:521)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
09-04 14:31:29.069: ERROR/AndroidRuntime(975):     at dalvik.system.NativeStart.main(Native Method)
09-04 14:31:29.099: INFO/Process(53): Sending signal. PID: 975 SIG: 3

It looks like your Cursor c is null. In your PlaceItemizedOverlay I see you have declared a private variable c but you have not assigned to it anywhere. The onClick code refers to this variable and will crash as above.

I guess you want to pass your Cursor in from your AboutMap Activity. Maybe add another parameter to the constructor of PlaceItemizedOverlay to share the cursor?

public PlaceItemizedOverlay(Drawable defaultMarker, Context context, Cursor cursor)
        {
            super(boundCenterBottom(defaultMarker));
            mContext = context;
            c = cursor;

        }