notifyDataSetChanged() NullPointerException : <p>i try to add onClick this button then create contact in database</p>

<p>Here is code i try to add it to main_fragment</p>

<pre><code>  final Button addBtn = (Button) view.findViewById(R.id.btnadd);
    addBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Uri imageUri = Uri.parse("android.resource://org.intracode.contactmanager/drawable/no_user_logo.png");
            import_fragment.Contact contact = new import_fragment.Contact(dbHandler.getContactsCount(), String.valueOf(nametxt.getText()), String.valueOf(phoneTxt.getText()), String.valueOf(emailTxt.getText()), String.valueOf(addressTxt.getText()), imageUri);
            if (!contactExists(contact)) {
                dbHandler.createContact(contact);
                Contacts.add(contact);
                contactAdapter.notifyDataSetChanged(); // Error in this line
                Toast.makeText(getActivity().getApplicationContext(), String.valueOf(nametxt.getText()) + " has been added to your Contacts!", Toast.LENGTH_SHORT).show();
                return;
            }
            Toast.makeText(getActivity().getApplicationContext(), String.valueOf(nametxt.getText()) + " already exists. Please use a different name.", Toast.LENGTH_SHORT).show();
        }
    });
</code></pre>

<p>When i press this button in my app, 'app has stopped working'</p>

<p>Here is my logcat</p>

<pre><code>01-22 08:31:04.014 29398-29398/com.al3almya.users.al3almya E/AndroidRuntime: FATAL EXCEPTION: main
                                                                         Process: com.al3almya.users.al3almya, PID: 29398
                                                                         java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.notifyDataSetChanged()' on a null object reference
                                                                             at com.al3almya.users.al3almya.main_fragment$1.onClick(main_fragment.java:77)
                                                                             at android.view.View.performClick(View.java:4848)
                                                                             at android.view.View$PerformClick.run(View.java:20262)
                                                                             at android.os.Handler.handleCallback(Handler.java:815)
                                                                             at android.os.Handler.dispatchMessage(Handler.java:104)
                                                                             at android.os.Looper.loop(Looper.java:194)
                                                                             at android.app.ActivityThread.main(ActivityThread.java:5637)
                                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                                             at java.lang.reflect.Method.invoke(Method.java:372)
                                                                             at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
                                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
</code></pre>
