java.lang.NullPointerException - canot find why

I'm little experimenting with FileConnection.
So far I put together this code

package browser;

import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.midlet.MIDlet;

public class MobileFileBrowser extends MIDlet{

private Display display = Display.getDisplay(this);
    public MobileFileBrowser(){}

public void startApp(){
        changeScreen(CoreUI.FILE_CONN_DETECTION);
    }

public void pauseApp(){}

public void destroyApp(boolean unconditional){}

public void quitApp(){
        destroyApp(true);
        notifyDestroyed();
    }

/**
     * Method to call classes as declared in CoreUI interface
     * @param _uiClass represent the name of the class that need to invoked
     */
    public void changeScreen(String _uiClass){
        System.out.println("Change screen with string "+ _uiClass);
        Displayable displayable;
        try{
            displayable = (Displayable)Class.forName(_uiClass).newInstance();
            display.setCurrent(displayable);
            ((CoreUI)displayable).setUIManager(this);
        }
        catch(Exception e){
            e.printStackTrace();
        }
    }

/**
     * Method provide way to display Alerts on the screen
     * @param d represent displayable (alert) to be presented to user
     */
    /*public void changeScreen(Displayable d){
        System.out.println("Change screen with displayable");
        display.setCurrent(d);
        ((CoreUI)d).setUIManager(this);
    } */
}

package browser.gui;

import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;
import javax.microedition.lcdui.Form;

import browser.MobileFileBrowser;
import browser.CoreUI;

public class FileConnDetection extends Form implements CoreUI{

private MobileFileBrowser mgr;

public FileConnDetection(){
        super("File Connection Detection");

if(System.getProperty(
                "microedition.io.file.FileConnection.version") != null){
            System.out.println("FileConnection API found calling "+CoreUI.FILE_BROWSER);
            mgr.changeScreen(CoreUI.FILE_BROWSER);
        }
        else{
            System.out.println("FileConnection API NOT found");
            StringBuffer deviceData = new StringBuffer(mgr.getAppProperty("MIDlet-Name"))
                    .append("\n").append(mgr.getAppProperty("MIDlet-Vendor"))
                    .append("\nFileConnection API is not available");
            Alert alert = new Alert("Missing API", deviceData.toString(), null, AlertType.INFO);
            alert.setTimeout(Alert.FOREVER);
           /* Display.getDisplay(mgr).setCurrent(alert);
            mgr.destroyApp(true);*/
        }
    }

public void setUIManager(MobileFileBrowser _mgr){
        mgr = _mgr;
    }
}

but when I run it I will get following errors

I think exception is because in MobileFileBrowser midlet changescreen method that you are calling from startApp, you are creating an instance of
of class browser.gui.FileConnDetection. Now in FileConnDetection constructor you are using mgr instance variable that has not been initialised yet.
So it is throwing null pointer exception.

Yes, I came to same conclusion. I was about to make some correction to code when I went little "off project" starting doing something else. Hopefully in week or two I will get back to it and post correction so other can have look at it.