Use if let instead of unwrap for nullable callback

ctru-rs/src/applets/swkbd.rs

@@ -869,12 +869,14 @@ impl SoftwareKeyboard {
 
         let mut retmsg = std::ptr::null();
 
-        swkbd.callback_result = extra.callback.unwrap()(
+        if let Some(cb) = extra.callback {
+            swkbd.callback_result = cb(
                 extra.callback_user,
                 &mut retmsg,
                 text8.as_ptr(),
                 text8.len(),
-        ) as _;
+            ) as _
+        };
 
         let retmsg = if !retmsg.is_null() {
             unsafe {