Fatal Exception: main and Null pointer exception arise while running a simple android program

I was trying to do some simple stuff on Android. I created a Login Page and worked on. When I try to create a pop up window, Fatal exception is thrown. Help out


public class NewProjectActivity extends Activity {

    PopupWindow popUp;
    LinearLayout layout;
    TextView tv;
    LayoutParams params;
    LinearLayout mainLayout;
    boolean click = true;

    protected void onCreate(Bundle savedInstanceState)  {
        TextView alertdetail = (TextView) findViewById(R.id.link_to_register);
        alertdetail.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if (click) {
                    popUp.showAtLocation(mainLayout, Gravity.BOTTOM, 10, 10);
                    popUp.update(50, 50, 300, 80);
                    click = false;
                } else {
                    click = true;

        params = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
        tv.setText("Hi this is a sample text for popup window");
        layout.addView(tv, params);
        popUp.showAtLocation(layout, Gravity.BOTTOM, 10, 10);
        mainLayout.addView(alertdetail, params);


    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.new_project, menu);
        return true;


TextView tv is not initialized and layout and popUp and mainLayout.

Also you have setContentView twice for the same activity which is not wrong but bad design

Your simple application is crashing because you have declared these variables :-

PopupWindow popUp;
LinearLayout layout;
TextView tv;
LayoutParams params;
LinearLayout mainLayout;

and initialised only :-

        TextView alertdetail = (TextView) findViewById(R.id.link_to_register);

This is called as passing the reference. You need to do the same for the above

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