I'm trying to make an android app but I encoutered this error and I don't know what to do and why is it showing from the first place, the app crashes with the following error in the log:

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.ramy.medicare, PID: 2487
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ramy.medicare/com.example.ramy.medicare.Profile.materialtabs.activity.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
                      at android.app.ActivityThread.-wrap12(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6119)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
                   
***Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                      at com.example.ramy.medicare.Profile.materialtabs.activity.MainActivity.onCreate(MainActivity.java:43)***

                      at android.app.Activity.performCreate(Activity.java:6679)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
                      at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:154) 
                      at android.app.ActivityThread.main(ActivityThread.java:6119) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

here is the code:

package com.example.ramy.medicare.Profile.materialtabs.activity;
 
  import android.content.Intent;
  import android.os.Bundle;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;
  import android.view.LayoutInflater;
  import android.view.View;
 
  import android.widget.Button;
 
  import com.example.ramy.medicare.R;
 
 
 
 
  public class MainActivity extends AppCompatActivity implements View.OnClickListener {
 
 
    private Button btnScrollableTabs_b, btnIconTabs_b, btnCustomIconTextTabs_b;
 
 
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
  //    toolbar = (Toolbar) findViewById(R.id.toolbar);
  //    setSupportActionBar(toolbar);
 
 
 
 
 
 
 
 
      btnScrollableTabs_b = (Button) findViewById(R.id.btnScrollableTabs);
      btnIconTabs_b = (Button) findViewById(R.id.btnIconTabs);
      btnCustomIconTextTabs_b = (Button) findViewById(R.id.btnCustomIconTabs);
 
      //at these lines is the error:
      **btnScrollableTabs_b.setOnClickListener(this); 
      btnIconTabs_b.setOnClickListener(this);
      btnCustomIconTextTabs_b.setOnClickListener(this);**
 
 
 
  //    btnSimpleTabs = (Button) findViewById(R.id.btnSimpleTabs);
  //    btnScrollableTabs = (Button) findViewById(R.id.btnScrollableTabs);
  //    btnIconTextTabs = (Button) findViewById(R.id.btnIconTextTabs);
  //    btnIconTabs = (Button) findViewById(R.id.btnIconTabs);
  //    btnCustomIconTextTabs = (Button) findViewById(R.id.btnCustomIconTabs);
 
  //    btnSimpleTabs.setOnClickListener(this);
 
  //    btnScrollableTabs.setOnClickListener(this);
  //    btnIconTextTabs.setOnClickListener(this);
  //    btnIconTabs.setOnClickListener(this);
  //    btnCustomIconTextTabs.setOnClickListener(this);
 
  }
 
    @Override
    public void onClick(View view) {
      switch (view.getId()) {
 
        case R.id.btnScrollableTabs:
          startActivity(new Intent(MainActivity.this, ScrollableTabsActivity.class));
          break;
        case R.id.btnIconTabs:
          startActivity(new Intent(MainActivity.this, IconTabsActivity.class));
          break;
        case R.id.btnCustomIconTabs:
          startActivity(new Intent(MainActivity.this, CustomViewIconTextTabsActivity.class));
          break;
      }
    }
  }

Kindly tell me what to do Im so confused, I instantiated the object so why am i getting this error ???