Merged PR 164: Clear the leftover TODOs

- Remove nullable type from container.properties.parentId
- Add Swal when trying to delete main container
- Moved default editor state to default.ts
- Moved default symbol model to default.ts

src/Interfaces/IContainerProperties.ts

@@ -8,9 +8,8 @@ export default interface IContainerProperties {
   /** id of the container */
   id: string
 
-  // TODO: replace null by empty string
   /** id of the parent container (null when there is no parent) */
-  parentId: string | null
+  parentId: string
 
   /** id of the linked symbol ('' when there is no parent) */
   linkedSymbolId: string