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The exception you getting is that you: Cannot add a null child view to a ViewGroup.

A ViewGroup is a view that can contain other views. The ViewGroup is the base class for Layouts in android, like LinearLayout , RelativeLayout , FrameLayout etc. In other words, ViewGroup is generally used to define the layout in which views(widgets) will be set/arranged/listed on the android screen.

You have two ViewGroups: one activity_main.xml and foundnum.xml

Within your code (MainActivity), you set activity_main.xml as your viewGroup by this line of code setContentView(R.layout.activity_main); in your oncreate method.

What causes the error?? The error is caused by taking action(setting text or setting onclick action) on tvfoundnum and buttonrestart1:

• TextView tvfoundnum= findViewById(R.id.tvfound)
• buttonrestart1 = (Button)findViewById(R.id.buttonrestart1);

Any action you set to them will result into an error because those views are not part of the set ViewGroup. In otherwards they are not contained in activity_main.xml and they are not yet known

Solution Options: There are 2 ways you can be able to achieve what you want:

1. By hiding and displaying only the views you want to display: Meaning you will have to combine foundnum.xml's code to activity_main.xml. Such that you have one layout.
2. By using fragments. With fragments, you can have one fragment manage more than one ViewGroup (layouts).

Since you are using activity, I will demonstrate solution option (1) bellow

Solution Option One. This is the new layout(it contains code from activity_main.xml & foundnum.xml): activity_main.xml.