Fix: Add collection declaration and link to App.java

[AyeongKwon]

I add collection declaration in IntroScreen.java.
To link to App.java, I modified some codes in it.

It works well before step into editing phase.
I don't know what is wrong.
Please, give me help....Thank you in advance.

[ljseop1030]

I think there was error due to the sc.close() in my InputHandler file.
sc.close() happened multiple times when the process is altogether with the other files.
I erased sc.close() and merged it a few minutes ago, so maybe it is resolved now.

[Kimhyewon0621]

Jiseop solve that problem!! GOOD

[mingyeonggg]

Good~

[AyeongKwon]

Thank you for your helps!!!
It is resolved!
I will be merge it now!😘😘