Namespace not specified. Specify a namespace in the module's build file. の調査

* What went wrong:
A problem occurred evaluating root project 'android'.
> A problem occurred configuring project ':app'.
   > Could not create an instance of type com.android.build.api.variant.impl.ApplicationVariantImpl.
      > Namespace not specified. Specify a namespace in the module's build file. See https://d.android.com/r/tools/upgrade-assistant/set-namespace for information about setting the namespace.