If I remove one i get theese errors:
C:/Users/Edvinas/Documents/InteractiveHelloWorld/main.cpp:2: In file included from main.cpp:2:

C:/Users/Edvinas/Documents/InteractiveHelloWorld/mainwindow.h:24: error: expected unqualified-id before '{' token

C:/Users/Edvinas/Documents/InteractiveHelloWorld/main.cpp:10: error: expected '}' at end of input

C:/Users/Edvinas/Documents/InteractiveHelloWorld/main.cpp:10: error: expected unqualified-id at end of input