java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference

i alway get an Error in my Android Project.

04-04 00:02:31.793  30166-30166/de.exayte.poolminer E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: de.exayte.poolminer, PID: 30166
    java.lang.RuntimeException: Unable to start activity ComponentInfo{de.exayte.poolminer/de.exayte.poolmonitor.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2329)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2391)
            at android.app.ActivityThread.access$900(ActivityThread.java:147)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1296)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5256)
            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:898)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:117)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference
            at de.exayte.poolmonitor.FragmentStart.onCreateView(FragmentStart.java:56)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1786)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
            at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:548)
            at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1220)
            at android.app.Activity.performStart(Activity.java:5949)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2292)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2391)
            at android.app.ActivityThread.access$900(ActivityThread.java:147)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1296)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5256)
            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:898)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
            at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:117)

In my MainActivity i'm calling the class FragmentStart.

FragmentStart.class

package de.exayte.poolmonitor;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;

public class FragmentStart extends Fragment {

    //Define Listview
    private ListView startList;
    private String[] stringList;
    Activity activity = getActivity();

    //Placeholder ?
    public FragmentStart() {
        //
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_start, container, false);

        // Create ListView start
        startList = (ListView) rootView.findViewById(R.id.startList);
        stringList = getResources().getStringArray(R.array.startList);

        if (activity != null) {
            ArrayAdapter<String> objAdapter = new ArrayAdapter<String>(activity, android.R.layout.simple_list_item_1, stringList);
            startList.setAdapter(objAdapter);
            startList.setOnItemClickListener(new AdapterView.OnItemClickListener() {

                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                    switch (startList.getPositionForView(view)) {

                        case 0:
                            // ...
                            break;

                    }
                }
            });
        }
        // Create ListView end

        //New Entry
        Button add = (Button) activity.findViewById(R.id.add);
        add.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                Intent myIntent = new Intent(view.getContext(), addItem.class);
                startActivityForResult(myIntent, 0);
            }

        });

        return rootView;
    }
}

fragment_start.xml

<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
xmlns:tools="http://ift.tt/LrGmb4"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="de.exayte.poolmonitor.FragmentStart">

<ListView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/startList"
    android:layout_gravity="center"
    android:layout_below="@+id/add"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/add"
    android:id="@+id/add"
    android:layout_alignParentTop="true"
    android:layout_toLeftOf="@+id/delete"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="false" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/delete"
    android:id="@+id/delete"
    android:layout_alignParentTop="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />

settings.xml

<?xml version="1.0" encoding="utf-8"?>

<string-array name="startList">

<item>lolo</item>

</string-array>

It would be awsome, if somone can help me! Greetings, eXayte