Keep the project appcompat_v7 opened

If you New a Android Project in Android-Eclipse using android.support.v7.app, an extra project appcompat_v7 will be created automatically, if it is not exist. And, if you mis-think that it is un-related and close the project appcompat_v7, you project will have Errors occurred during the build.

Errors running builder 'Android Pre Compiler' on project 'xxx'.
java.lang.NullPointerException

and/or

R cannot be resolved to a variable.

If your project have to import android.support.v7.app. just keep the project appcompat_v7 opened.