Gradle is not closing the port after stop the application

In Java, I am using Spring and Gradle and having the following issue (that I can't find the answer for!)

I am getting this error again and again when trying to compile...

   FAILURE: Build failed with an exception.

    What went wrong:
    Execution failed for task ':bootRun'.
    > Process 'command 'C:\Program Files\Java\jdk1.8.0_131\bin\java.exe'' finished with non-zero exit value 1

    Try:
    Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

MORE CLUES:

• The first time I open intelliJ, it works. Then it doesn't work again
• I am closing the app properly (click the red square), it is not that.
• The process using the port 80 is "java", so java is listening correctly...
• ...but it remains open even after closing the app
• If I close and open intelliJ, it works, but after stop the app is says:

The daemon will cancel the build. Could not execute build using Gradle distribution 'https://services.gradle.org/distributions/gradle-3.5-rc-2-all.zip'. 12:57:48: External task execution finished 'bootRun'

Seems like Gradle is not closing properly my app, and it only works one time, then the port remains open forever even if I click stop (well, until I close IntelliJ!).

Here is my project in Github:

https://github.com/RicSala/project5countries

I have red EVERYTHING out there to fix it, but nothing helps...

Please help! :)

THANK YOU!