title: getActivity() return null in Fragment
body: <p>When I change orientation, and swipe, it crashes.
  This is my log cat</p>
  
  <pre><code>11-01 11:21:19.715: E/AndroidRuntime(4336): FATAL EXCEPTION: main 
  11-01 11:21:19.715: E/AndroidRuntime(4336): java.lang.NullPointerException
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at com.example.saisuke.CalendarAdapter.&lt;init&gt;(CalendarAdapter.java:33)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at com.example.saisuke.MonthFragment.updateCurrentMonth(MonthFragment.java:161) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at com.example.saisuke.MonthFragment.onNextMonth(MonthFragment.java:177)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at com.example.saisuke.Saisuke$1.onPageScrollStateChanged(Saisuke.java:107) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.support.v4.view.ViewPager.setScrollState(ViewPager.java:398)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.support.v4.view.ViewPager.access$000(ViewPager.java:84) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.support.v4.view.ViewPager$3.run(ViewPager.java:243) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.view.Choreographer.doCallbacks(Choreographer.java:555) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.view.Choreographer.doFrame(Choreographer.java:524)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.os.Handler.handleCallback(Handler.java:615) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.os.Handler.dispatchMessage(Handler.java:92) 11-01 
  11:21:19.715: E/AndroidRuntime(4336):   at android.os.Looper.loop(Looper.java:137)
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at android.app.ActivityThread.main(ActivityThread.java:4898) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at java.lang.reflect.Method.invokeNative(Native Method) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at java.lang.reflect.Method.invoke(Method.java:511) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1006) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773) 
  11-01 11:21:19.715: E/AndroidRuntime(4336):     at dalvik.system.NativeStart.main(Native Method)
  </code></pre>
  
  <p>MainActivity.java</p>
  
  <pre><code>@Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_saisuke);
          ((Global) this.getApplication()).setStraightMode(false);
          mOldState = ((Global) this.getApplication()).getStraightMode();
          monthList = new ArrayList&lt;MonthFragment&gt;(3);
          curMonth = Calendar.getInstance();
          Calendar prevMonth, nextMonth;
          prevMonth = (Calendar) curMonth.clone();
          nextMonth = (Calendar) curMonth.clone();
          prevMonth.set(Calendar.MONTH, prevMonth.get(Calendar.MONTH) - 1);
          nextMonth.set(Calendar.MONTH, nextMonth.get(Calendar.MONTH) + 1);
              monthList.add(MonthFragment.newInstance(prevMonth));
              monthList.add(MonthFragment.newInstance(curMonth));
              monthList.add(MonthFragment.newInstance(nextMonth));
          //tmpCurMonth = monthList[1].getCurCalendar();
          mViewPager = (ViewPager) findViewById(R.id.pager);
          mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager(), monthList);
          mViewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
              @Override
              public void onPageScrollStateChanged(int arg0) {
                  // TODO Auto-generated method stub
                  if (arg0 == ViewPager.SCROLL_STATE_IDLE) {
                      if (mSelectedPageIndex &lt; PAGE_MIDDLE) {
                          monthList.get(0).onPreviousMonth();
                          monthList.get(1).onPreviousMonth();
                          monthList.get(2).onPreviousMonth();  
                      } else if (mSelectedPageIndex &gt; PAGE_MIDDLE) {
                          monthList.get(0).onNextMonth();
                          monthList.get(1).onNextMonth();
                          monthList.get(2).onNextMonth();
                      }
                      mViewPager.setCurrentItem(1, false);
                      //tmpCurMonth = monthList[1].getCurCalendar();
                      mSectionsPagerAdapter.notifyDataSetChanged();
                  }
              }
  
              @Override
              public void onPageScrolled(int arg0, float arg1, int arg2) {
                  // TODO Auto-generated method stub
                  if (((Global) getApplication()).getStraightMode() != mOldState){
                      mOldState = ((Global) getApplication()).getStraightMode();
                      mSectionsPagerAdapter.notifyDataSetChanged();   
                  }
              }
  
              @Override
              public void onPageSelected(int arg0) {
                  // TODO Auto-generated method stub
                  mSelectedPageIndex = arg0;
              }
          });
          mViewPager.setAdapter(mSectionsPagerAdapter);
          mViewPager.setCurrentItem(1, false);
      }
  </code></pre>
  
  <p>MyFragent.java</p>
  
  <pre><code>static MonthFragment newInstance(Calendar a) {
          MonthFragment f = new MonthFragment(a);
          return f;    
      }
  
  @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
              Bundle savedInstanceState) {
  
          if (savedInstanceState != null) {
              // Restore last state
              calendar = (Calendar) savedInstanceState.getSerializable("starttime");
          }
  
          mMainView = inflater.inflate(R.layout.fragment_saisuke, container, false);
          adap = new CalendarAdapter(getActivity(), calendar);
          ....        
          return mMainView;
      }
  
  protected void updateCurrentMonth() {
          adap.updateData();
      }
  
  protected final void onNextMonth() {
          if (calendar.get(Calendar.MONTH) == Calendar.DECEMBER) {
              calendar.set((calendar.get(Calendar.YEAR) + 1), Calendar.JANUARY, 1);
          } else {
              calendar.set(Calendar.MONTH, calendar.get(Calendar.MONTH) + 1);
          }
          updateCurrentMonth();
      }
  </code></pre>