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)