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/Components/ElementsSidebar/ElementsSidebar.test.tsx

@@ -15,7 +15,7 @@ describe.concurrent('Elements sidebar', () => {
         parent: null,
         properties: {
           id: 'main',
-          parentId: null,
+          parentId: '',
           linkedSymbolId: '',
           displayedText: 'main',
           x: 0,