fix: prevent clearFadeHold undefined error

Author: charlieforward9

src/source/source_cache.ts

@@ -676,7 +676,9 @@ export class SourceCache extends Evented {
         for (const retainedId in retain) {
             // Make sure retained tiles always clear any existing fade holds
             // so that if they're removed again their fade timer starts fresh.
-            this._tiles[retainedId].clearFadeHold();
+            if (this._tiles[retainedId]) {
+                this._tiles[retainedId].clearFadeHold();
+            }            
         }
 
         // Remove the tiles we don't need anymore.