Build directory color does not update after building

1. Open a new qmake-based project.
   In "Projects" -> "Build Settings", the build directory is written in red, because it does not exist.
2. Build the project.
   The build directory is still written in red, although it does exist now.
3. Change the string in the build directory edit to some other value and back to the previous one.
   Now the string turns black.

The text should turn black as soon as the project is being built.