Android Studio - Cannot Drag And Drop Button

I just created a new project, blank activity. If I go into the interface for the activity_main.xml I cannot drag and drop any widgets. I tried changing the theme, still nothing. He

Solution 1:

Android Studio changed the layout activity_main.xml and made it as a container for things like FloatingActionButton, ToolBar.. so you can't drag and drop buttons there. They created another xml file called content_main.xml where you can add all your other views. They include the content_main inside activity_main by

<include layout="@layout/content_main" />

Solution 2:

I noticed that adding something programmatically to the xml file (click "text" button on the bottom left) then made it so the ui worked... but only after I added something programmatically.