After developing an Angular application, everything seemed to be functioning well when I included the controllers.js, routes.js, directives.js files separately in index.html. However, upon attempting to combine these files into a single js file using gulp-concat (following the same order as in my HTML), I started encountering the error message: Angular Uncaught Error: [$injector:modulerr]. The vague nature of this error has left me uncertain about the next steps to take.