Merge pull request #3205 from dreampiggy/bugfix_session_invalid

Ensure the invalid URLSeesion will receive the cancel callback and does not trigger the any exception from framework

SDWebImage/Core/SDWebImageDownloader.m

@@ -140,11 +140,12 @@ static void * SDWebImageDownloaderContext = &SDWebImageDownloaderContext;
 }
 
 - (void)dealloc {
-    [self.session invalidateAndCancel];
-    self.session = nil;
-    
     [self.downloadQueue cancelAllOperations];
     [self.config removeObserver:self forKeyPath:NSStringFromSelector(@selector(maxConcurrentDownloads)) context:SDWebImageDownloaderContext];
+    
+    // Invalide the URLSession after all operations been cancelled
+    [self.session invalidateAndCancel];
+    self.session = nil;
 }
 
 - (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations {

SDWebImage/Core/SDWebImageDownloaderOperation.m

@@ -194,6 +194,13 @@ typedef NSMutableDictionary<NSString *, id> SDCallbacksDictionary;
             }
         }
         
+        if (!session.delegate) {
+            // Session been invalid and has no delegate at all
+            [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:SDWebImageErrorInvalidDownloadOperation userInfo:@{NSLocalizedDescriptionKey : @"Session delegate is nil and invalid"}]];
+            [self reset];
+            return;
+        }
+        
         self.dataTask = [session dataTaskWithRequest:self.request];
         self.executing = YES;
     }