setter中的java意外NullpointExpection

Question

我试图做一个简单的操作：点击一个按钮并显示一个自定义的DialogFragment，但是我得到了一个NullPointExpection，我不知道为什么

mAlertDialog。java:

public class mAlertDialog extends DialogFragment {

    TextView title;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.dialog_invalid_time, container, false);
        title = (TextView)view.findViewById(R.id.titleText);
        return view;
    }

    public void setTheTitle(String title) {
        this.title.setText(title);
    }
}

显示mAlertDialog:

mAlertDialog dialog = new mAlertDialog();
dialog.setTheTitle(getActivity().getString(R.string.invalidTimeTitle));
dialog.show(fm, "InvalidTime");

错误消息：

java.lang.NullPointerException: Attempt to invoke virtual method 'void 安卓.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
    at m.inschool8.Objects.mAlertDialog.setTheTitle(mAlertDialog.java:20)
    at m.inschool8.bSubjects.Fragment_Subjects$55.onClick(Fragment_Subjects.java:2654)
    at 安卓.view.View.performClick(View.java:4780)
    at 安卓.view.View$PerformClick.run(View.java:19866)
    at 安卓.os.Handler.handleCallback(Handler.java:739)
    at 安卓.os.Handler.dispatchMessage(Handler.java:95)
    at 安卓.os.Looper.loop(Looper.java:135)
    at 安卓.app.ActivityThread.main(ActivityThread.java:5254)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
    at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:698)