The problem's been solved.

I just replaced

TFT_eSPI_ext tft = TFT_eSPI_ext();

By

TFT_eSPI tftExt; // Create the base TFT_eSPI instance
TFT_eSPI_ext tft(&tftExt); // Pass a pointer to the base instance

Also declarations of Screens after directives and before the setup function were added and everything works now.