Receving this error after registration

[marmik57]

main
java.lang.NullPointerException
at com.smartwaysolutions.chatapplication.ui.adapters.UserListingRecyclerAdapter.onBindViewHolder(UserListingRecyclerAdapter.java:37)
at com.smartwaysolutions.chatapplication.ui.adapters.UserListingRecyclerAdapter.onBindViewHolder(UserListingRecyclerAdapter.java:15)
at android.support.v7.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:6356)
at android.support.v7.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:6389)
at android.support.v7.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:5335)
at android.support.v7.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:5598)
at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5440)
at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5436)
at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2224)
at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1551)
at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1511)
at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:595)
at android.support.v7.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3583)
at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:3312)
at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:3844)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.support.v4.widget.SwipeRefreshLayout.onLayout(SwipeRefreshLayout.java:636)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.support.v4.view.ViewPager.onLayout(ViewPager.java:1795)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1649)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1507)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1420)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.support.design.widget.HeaderScrollingViewBehavior.layoutChild(HeaderScrollingViewBehavior.java:131)
at android.support.design.widget.ViewOffsetBehavior.onLayoutChild(ViewOffsetBehavior.java:42)
at android.support.design.widget.AppBarLayout$ScrollingViewBehavior.onLayoutChild(AppBarLayout.java:1391)
at android.support.design.widget.CoordinatorLayout.onLayout(CoordinatorLayout.java:870)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1649)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1507)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1420)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1649)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1507)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1420)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:4380)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:13836)
at android.view.ViewGroup.layout(ViewGroup.java:

[WaqasAzamKml]

The code i'm posting is not the optimal solution, but it'll handle this error.
Update your onBindViewHolder inside UserListingRecyclerAdapter.java file

@Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        User user = mUsers.get(position);
        String alphabet;
        if(user.email!=null) {
            alphabet = user.email.substring(0, 1);
            holder.txtUsername.setText(user.email);
            holder.txtUserAlphabet.setText(alphabet);
        }
        else {
            alphabet = "n";
            holder.txtUsername.setText("null@null.com");
            holder.txtUserAlphabet.setText(alphabet);
        }

    }