ConstraintLayout和工具栏的奇怪问题

Question

I've got some problems. with Costraint Layout and Toolbar. I upload some images to explain better what happens. I can't do nothing. If I add a new button It doesn't show it. I don't know if it is a normal situation, but if you have a solution, please, help me. Thanks in advance

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Answer 1

在xml文件中添加button ，并告诉我们如果它不起作用将导致什么错误

Answer 2

Change your ConstraintLayout to RelativeLayout and then add below code and see what happens:

android:layout_centerHorizontal="true"
android:layout_centerVertical="true"

Answer 3

在gradle文件中将android api目标更改为25