conflict between app.js and leaflet causes a grey screen instead of map

Hi, this problem is making me crazy...

I use OpenStreetMap and leaflet to show a map in a form, in order to get the user's coordinates. but it only shows a blank grey screen.

when I delete app.js script tag from the page, it works fine and the map shows up.

I have searched a lot to solve this problem, but nothing.

I really appreciate it guys if you take a look at it: https://niki24.ir/donators/register

I have done the getting started guide in the following URL: https://switch2osm.org/using-tiles/getting-started-with-leaflet/