Invalid markup syntax will lead to crash

Describe the bug A clear and concise description of what the bug is.

To Reproduce Steps to reproduce the behavior:

1. Go to homepage https://idyll-lang.org/
2. Click on the editable area and start to input []()
3. See error

Expected behavior render []() as text. or maybe just warn user it's not valid

Screenshots N/A

Desktop (please complete the following information):

• OS: Mac
• Browser chrome
• Version 87

Additional context N/A