jQuery找不到加载的输入

I have a little problem, with the jquery .load() function. This is my code:

$("body").on("click", ".popup-newPost", function(event) {
    $( ".popup" ).load( "/parts/popup/newPost.php" );

    if ($('input#termin-or-post').is(':checked')) {
        alert("test");
    }
});

When I load my content into the .popup div, everything works pretty good, but when I want to check, if the user checked the checkbox, which was loaded in my .popup before, it doesn't work (I think jquery can't find the object... "why ?")

I tried to change the text in a loaded input field (type=text) - some reaction - nothing :(

Greets

.load is an async function, use the callback!

$( ".popup" ).load( "/parts/popup/newPost.php", function() {
    if ($('input#termin-or-post').is(':checked')) {
        alert("test");
    }
});