Design files and theme files are the key files of your project; thus the integrity of these files are critical. Issues may occur in these files in the following cases:
If you merge another branch into yours and forget to resolve conflicts (if there are any).
If you edit theme files manually (which is not required most of the time and not recommended) and JSON file gets broken.
Due to their importance, these critical files are continuously being watched and analyzed. If something happens that may cause issues in your project, you can what is wrong in a panel called Problems Panel.
For some reason, it might be required to restart the TypeScript watcher.
Go to the menu on the top
And choose Run Task under Terminal
From the panel, choose the selected item in the screenshot which triggers the watcher