Fix the symbol cache and image cache

PortfolioManager/Cache/ImageCache.cs

@@ -57,6 +57,7 @@ namespace PortfolioManager.Cache
     {
       lock (thisLock)
       {
+        DisposeBitmaps();
         imageCache = new Dictionary<ImageType, Bitmap>();
       }
     }
@@ -66,22 +67,30 @@ namespace PortfolioManager.Cache
       lock (thisLock)
       {
         if (null == imageCacheInstance) return;
-        List<Bitmap> bitmaps = imageCache.Values.ToList();
-        foreach (Bitmap bitmap in bitmaps)
-        {
-          bitmap.Dispose();
-        }
+        DisposeBitmaps();
         imageCache = null;
         imageCacheInstance = null;
       }
     }
-
-    public IImage GetImage(ImageCache.ImageType imageType)
+    
+    private void DisposeBitmaps()
     {
-      lock(this)
+      if(null == imageCache)return;
+      List<Bitmap> bitmaps = imageCache.Values.ToList();
+      foreach (Bitmap bitmap in bitmaps)
       {
-        return imageCache[imageType];
+        bitmap.Dispose();
       }
     }
+
+    public IImage GetImage(ImageType imageType)
+    {
+        lock(thisLock)
+        {
+          if (imageCache == null)throw new ObjectDisposedException(nameof(ImageCache));
+          if (!imageCache.TryGetValue(imageType, out var bitmap))throw new KeyNotFoundException($"Image {imageType} not found in cache.");
+          return bitmap;
+        }
+    }    
   }
 }