How to solve null pointer exception when working with Firebase

Question

I am getting a null pointer exception while trying to retrieve data using Firebase.

The following is a description of my error:-

The error was pointing on myRef = mfirebaseDatabase.getReference();

This is my full error:-

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.vignanstudent/com.example.vignanstudent.v_b_d}: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.DatabaseReference com.google.firebase.database.FirebaseDatabase.getReference()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3335)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3474)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2081)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7643)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.DatabaseReference com.google.firebase.database.FirebaseDatabase.getReference()' on a null object reference
        at com.example.vignanstudent.v_b_d.onCreate(v_b_d.java:39)
        at android.app.Activity.performCreate(Activity.java:7815)
        at android.app.Activity.performCreate(Activity.java:7804)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1318)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3310)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3474) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2081) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7643) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)