XML : Android - Binary XML file line #19: Error inflating class fragment

my android app started giving me this (logcat) error after start.

My Main Activity is:

  import android.app.Activity;  import android.support.v7.app.AppCompatActivity;  import android.support.v7.app.ActionBar;  import android.support.v4.app.Fragment;  import android.support.v4.app.FragmentManager;  import android.os.Bundle;  import android.view.LayoutInflater;  import android.view.Menu;  import android.view.MenuItem;  import android.view.View;  import android.view.ViewGroup;  import android.support.v4.widget.DrawerLayout;    public class MainActivity extends AppCompatActivity          implements my.app.NavigationDrawerFragment.NavigationDrawerCallbacks {        /**       * Fragment managing the behaviors, interactions and presentation of the navigation drawer.       */      private my.app.NavigationDrawerFragment mNavigationDrawerFragment;        /**       * Used to store the last screen title. For use in {@link #restoreActionBar()}.       */      private CharSequence mTitle;        @Override      protected void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          setContentView(R.layout.activity_main);            mNavigationDrawerFragment = (my.app.NavigationDrawerFragment)                  getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);          mTitle = getTitle();            // Set up the drawer.          mNavigationDrawerFragment.setUp(                  R.id.navigation_drawer,                  (DrawerLayout) findViewById(R.id.drawer_layout));      }        @Override      public void onNavigationDrawerItemSelected(int position) {            Fragment myFragment = null;            switch (position) {                case 0:                  myFragment = new First_Fragment();                  break;                case 1:                  myFragment = new Second_Fragment();                  break;                case 2:                  myFragment = new Third_Fragment();                  break;                case 3:                  myFragment = new Fourth_Fragment();                  break;              //case 3:              //    myFragment = new Caseopen_Fragment();              //    break;          }          // update the main content by replacing fragments          FragmentManager fragmentManager = getSupportFragmentManager();          fragmentManager.beginTransaction()                  .replace(R.id.container, myFragment)                  .commit();      }        public void onSectionAttached(int number) {          switch (number) {              case 1:                  mTitle = getString(R.string.title_section1);                  break;              case 2:                  mTitle = getString(R.string.title_section2);                  break;              case 3:                  mTitle = getString(R.string.title_section3);                  break;              case 4:                  mTitle = getString(R.string.title_section4);                  break;              //case 4:              //    mTitle = getString(R.string.title_caseopen);              //    break;          }      }        public void restoreActionBar() {          ActionBar actionBar = getSupportActionBar();          actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);          actionBar.setDisplayShowTitleEnabled(true);          actionBar.setTitle(mTitle);      }          @Override      public boolean onCreateOptionsMenu(Menu menu) {          if (!mNavigationDrawerFragment.isDrawerOpen()) {              // Only show items in the action bar relevant to this screen              // if the drawer is not showing. Otherwise, let the drawer              // decide what to show in the action bar.              getMenuInflater().inflate(R.menu.main, menu);              restoreActionBar();              return true;          }          return super.onCreateOptionsMenu(menu);      }        @Override      public boolean onOptionsItemSelected(MenuItem item) {          // Handle action bar item clicks here. The action bar will          // automatically handle clicks on the Home/Up button, so long          // as you specify a parent activity in AndroidManifest.xml.          //int id = item.getItemId();            //noinspection SimplifiableIfStatement         // if (id == R.id.action_settings) {         //     return true;         // }            return super.onOptionsItemSelected(item);      }        /**       * A placeholder fragment containing a simple view.       */      public static class PlaceholderFragment extends Fragment {          /**           * The fragment argument representing the section number for this           * fragment.           */          private static final String ARG_SECTION_NUMBER = "section_number";            /**           * Returns a new instance of this fragment for the given section           * number.           */          public static PlaceholderFragment newInstance(int sectionNumber) {              PlaceholderFragment fragment = new PlaceholderFragment();              Bundle args = new Bundle();              args.putInt(ARG_SECTION_NUMBER, sectionNumber);              fragment.setArguments(args);              return fragment;          }            public PlaceholderFragment() {          }            @Override          public View onCreateView(LayoutInflater inflater, ViewGroup container,                                   Bundle savedInstanceState) {              View rootView = inflater.inflate(R.layout.fragment_main, container, false);              return rootView;          }            @Override          public void onAttach(Activity activity) {              super.onAttach(activity);              ((MainActivity) activity).onSectionAttached(                      getArguments().getInt(ARG_SECTION_NUMBER));          }      }    }    

I googled a lot, only thing, that I found (use extend FragmentActivity) but then my restoreActionBar() does not work. Android studio does not show any errors, before I run the app.

Any s