Deffer updateAnnotationTiles() if Source is not available

The Style object might have been created but the Source is not
yet available because the JSON file was not set. Can happen
specially when the style is loaded from the network.

src/mbgl/map/map_context.cpp

@@ -150,11 +150,20 @@ void MapContext::updateAnnotationTiles(const std::unordered_set<TileID, TileID::
     util::exclusive<AnnotationManager> annotationManager = data.getAnnotationManager();
     annotationManager->markStaleTiles(ids);
 
-    if (!style) return;
+    if (!style) {
+        return;
+    }
 
     // grab existing, single shape annotations source
     const auto& shapeID = AnnotationManager::ShapeLayerID;
-    style->getSource(shapeID)->enabled = true;
+    Source* shapeAnnotationSource = style->getSource(shapeID);
+
+    // Style not parsed yet
+    if (!shapeAnnotationSource) {
+        return;
+    }
+
+    shapeAnnotationSource->enabled = true;
 
     // create (if necessary) layers and buckets for each shape
     for (const auto &shapeAnnotationID : annotationManager->getOrderedShapeAnnotations()) {