Can not paste image from clipboard

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

I can not paste an image from clipboard, which outputs the following errors:
```
index.e9b7d4f516d61d3c8942.js:209 Uncaught TypeError: Cannot read properties of null (reading 'type')
at t.value (index.e9b7d4f516d61d3c8942.js:209)
at t.value (index.e9b7d4f516d61d3c8942.js:209)
at t.value (index.e9b7d4f516d61d3c8942.js:209)
at t.value (index.e9b7d4f516d61d3c8942.js:209)
at t.value (index.e9b7d4f516d61d3c8942.js:209)
at index.e9b7d4f516d61d3c8942.js:209
at z (eval at e.exports (index-common.4599673c9654fa0ae31a.js:1), :1:3612)
at O (eval at e.exports (index-common.4599673c9654fa0ae31a.js:1), :1:3735)
at HTMLTextAreaElement.eval (eval at e.exports (index-common.4599673c9654fa0ae31a.js:1), :5:25945)
```

The edit panel doesn't show anything.
However, I found it works fine in Firefox (in the same OS).

#### Current behaviour

> Screenshot (optional)

Provide steps to reproduce:

1. Screenshot an image with ctrl+shift+printscreen
2. Paste it to the edit panel
3. See error

I encountered this problem even it is a blank note.

#### Environment

I use HackMD on:

**Desktop**
- OS: Ubuntu 20.04 (Desktop)
- Browser: Chrome Version 93.0.4577.63 (Official Build) (64-bit)

Riproduci il problema in Chrome 93 su Ubuntu 20.04 usando lo screenshot e i passaggi di incolla indicati, quindi ispeziona il pannello di modifica e la console del browser durante l’incollamento. Traccia la gestione dell’immagine negli appunti a partire dal TypeError segnalato e confronta il risultato con Firefox. Il lavoro è completato quando l’immagine appare in una nota vuota senza errori.

Scritto dal modello di indicizzazione a partire dal testo della issue.