Fix layer store bug due to eqeqeq strong comparison

The layer store had a comparison that didn't work anymore due to the
introduction of the eqeqeq rule. Need to check for undefined *and* null
separately now.

src/store/modules/layers.store.js

@@ -193,7 +193,7 @@ const getters = {
      * @returns {AbstractLayer | null} Active layer or null if the index is invalid
      */
     getActiveLayerByIndex: (state) => (index) => {
-        if (index < 0 || index === null) {
+        if (index < 0 || index === undefined || index === null) {
             throw new Error(`Failed to get ActiveLayer by index: invalid index ${index}`)
         }
         return state.activeLayers.at(index) ?? null