android - "Attempt to invoke virtual method on a null object reference" when trying to getLayoutParams of a CardView in a DialogFragment?

I'm trying to set the size of a
CardView inside of a DialogFragment,
but I get this
error:

java.lang.NullPointerException: Attempt to invoke virtual method
'android.view.ViewGroup$LayoutParams
android.support.v7.widget.CardView.getLayoutParams()' on a null object
reference

This is
my code:

 CardView
            cardView;
 onCreate(...){
 cardView = (CardView)
            findViewById(R.id.cardViewInDialogFragment);

            }


 private void openDialogFragment()
            {

 DisplayMetrics displayMetrics = new DisplayMetrics();

            getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);


            BlankFragment blankFragment = new BlankFragment();

            blankFragment.show(getSupportFragmentManager(),"blankFragment");


            CardView.LayoutParams layoutParams = (CardView.LayoutParams)
            cardView.getLayoutParams();
 layoutParams.height = displayMetrics.heightPixels
            - 20;

 layoutParams.width = displayMetrics.widthPixels -
            20;
 cardView.setLayoutParams(layoutParams);

            }

Is it not possible
or am I doing something
wrong?

UPDATE:
The
cardView is inside the
fragment.

UPDATE
2:

When changing the code to
bellow:

BlankFragment:

public
            static View view;
pulic void onCreateView(...) {

 view =
            inflater.inflate(R.layout.fragment_blank, container, false);


            return
            view;
}

MainActivity:

CardView
            cardView;
protected void onCreate(...){

            ...


 cardView = (CardView)
            BlankFragment.view.findViewById(R.id.cardViewInDialogFragment);

            openDialogFragment();
}

private void openDialogFragment()
            {

 DisplayMetrics displayMetrics = new DisplayMetrics();

            getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);


            BlankFragment blankFragment = new BlankFragment();


            blankFragment.show(getSupportFragmentManager(),"blankFragment");


            CardView.LayoutParams layoutParams = (CardView.LayoutParams)
            cardView.getLayoutParams();
 layoutParams.height = displayMetrics.heightPixels
            - 20;
 layoutParams.width = displayMetrics.widthPixels - 20;

            cardView.setLayoutParams(layoutParams);
}

,
I get this
error:

FATAL EXCEPTION: main
Process:
com.sim.buttombarnavigationandfragment, PID: 10685

java.lang.RuntimeException: Unable to start activity

ComponentInfo{com.sim.buttombarnavigationandfragment/com.sim.buttombarnavigationandfragment.MainActivity}:

java.lang.NullPointerException: Attempt to invoke virtual method

'android.view.View android.view.View.findViewById(int)' on a null
object
reference
at


android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2680)

at

android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2741)
at
android.app.ActivityThread.-wrap12(ActivityThread.java)
at

android.app.ActivityThread$H.handleMessage(ActivityThread.java:1488)
at
android.os.Handler.dispatchMessage(Handler.java:102)
at
android.os.Looper.loop(Looper.java:154)
at
android.app.ActivityThread.main(ActivityThread.java:6176)
at
java.lang.reflect.Method.invoke(Native Method)

at

com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:778)
Caused by:
java.lang.NullPointerException: Attempt to invoke virtual
method
'android.view.View android.view.View.findViewById(int)' on a
null object
reference
at

com.sim.buttombarnavigationandfragment.MainActivity.onCreate(MainActivity.java:56)

at android.app.Activity.performCreate(Activity.java:6679)

at


android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)

at

android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2633)

at

android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2741) 
at
android.app.ActivityThread.-wrap12(ActivityThread.java) 
at

android.app.ActivityThread$H.handleMessage(ActivityThread.java:1488) 
at
android.os.Handler.dispatchMessage(Handler.java:102) 
at
android.os.Looper.loop(Looper.java:154) 

at
android.app.ActivityThread.main(ActivityThread.java:6176) 
at
java.lang.reflect.Method.invoke(Native Method) 
at

com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888) 

at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:778)