Fix crash in egui_extras::FileLoader after forget_image

crates/egui/src/context.rs

@@ -3446,6 +3446,7 @@ impl Context {
     /// Release all memory and textures related to the given image URI.
     ///
     /// If you attempt to load the image again, it will be reloaded from scratch.
+    /// Also this cancels any ongoing loading of the image.
     pub fn forget_image(&self, uri: &str) {
         use load::BytesLoader as _;
 

crates/egui_extras/src/loaders/file_loader.rs

@@ -95,10 +95,14 @@ impl BytesLoader for FileLoader {
                             }
                             Err(err) => Err(err.to_string()),
                         };
-                        let prev = cache.lock().insert(uri.clone(), Poll::Ready(result));
-                        assert!(matches!(prev, Some(Poll::Pending)), "unexpected state");
-                        ctx.request_repaint();
-                        log::trace!("finished loading {uri:?}");
+                        let mut cache = cache.lock();
+                        if cache.contains_key(&uri) {
+                            cache.insert(uri.clone(), Poll::Ready(result));
+                            ctx.request_repaint();
+                            log::trace!("finished loading {uri:?}");
+                        } else {
+                            log::trace!("cancelled loading {uri:?}\nNote: This can happen if `forget_image` is called while the image is still loading.");
+                        }
                     }
                 })
                 .expect("failed to spawn thread");