fix: handle layers is not array after restore

Author: Akryum

Diff for: packages/app-frontend/src/features/timeline/composable/layers.ts

@@ -40,19 +40,17 @@ export function layerFactory(options: LayerFromBackend): Layer {
 
 export function getLayers(appId: string) {
   let layers = layersPerApp.value[appId]
-  if (!layers) {
-    layers = []
-    layersPerApp.value[appId] = layers
+  if (!layers || !Array.isArray(layers)) {
+    layersPerApp.value[appId] = []
     layers = layersPerApp.value[appId]
   }
   return layers
 }
 
 function getHiddenLayers(appId: string) {
   let layers = hiddenLayersPerApp.value[appId]
-  if (!layers) {
-    layers = []
-    hiddenLayersPerApp.value[appId] = layers
+  if (!layers || !Array.isArray(layers)) {
+    hiddenLayersPerApp.value[appId] = []
     layers = hiddenLayersPerApp.value[appId]
   }
   return layers