error with MSS620
<!-- You must use the issue template below when submitting a bug -->

When I add the MSS620 device I start to get this errors:

```
[11/3/2020, 8:12:36 PM] [outdoor1] MSS620 turned Off
(node:17845) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'digest' of undefined
    at Meross.getOnCharacteristicHandler (/usr/lib/node_modules/homebridge-meross/src/index.ts:352:34)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
(node:17845) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 14)
```

<!-- A clear and concise description of what the bug is. -->

add this in the config

 ```
      {
            "model": "MSS620",
            "name": "outdoor1",
            "deviceUrl": "192.168.1.78",
            "channel": 1,
            "messageId": "75eeaa0f086ac8e12a62420cff9eba82",
            "timestamp": 1591348163,
            "sign": "92db6bb658f43631796f6a8215b29957",
            "accessory": "Meross"
        }
```

<!-- Steps to reproduce the behavior. -->

as other devices

<!-- A clear and concise description of what you expected to happen. -->

**Logs:**

<!-- Bug reports that do not contain logs may be closed without warning. -->

```
Show the Homebridge logs here. If using Homebridge Config-UI-X, you can easly downlaod your log by visiting the log page on the UI and Clicking the Download Icon in the top right.
Remove any sensitive information, such as yourhomebridge-meross timestamp / sign / messageId / deviceUrl.

Please Turn on Debug Mode before submitting your Issue so more logs can be shared to fix the issue being submitted.

***Read and Paste over this Text the Text Above^^^^^^
```

**Homebridge Config:**

```json
{
    "bridge": {
        "name": "Homebridge 69D4",
        "username": "xxxxx",
        "port": 51787,
        "pin": "814-14-416"
    },
    "accessories": [
        {
            "model": "MSS310",
            "name": "angoliera",
            "deviceUrl": "192.168.1.133",
            "channel": 0,
            "messageId": "2a7cf801ccb1b2690f882006b1ec7306",
            "timestamp": 1591348077,
            "sign": "3045b761bc2a59b19fa3965dfc9053a3",
            "accessory": "Meross"
        },
        {
            "model": "MSS210",
            "name": "studio",
            "deviceUrl": "192.168.1.15",
            "channel": 0,
            "messageId": "2a7cf801ccb1b2690f882006b1ec7306",
            "timestamp": 1591348163,
            "sign": "bf042f4979c261d9480da799ea8a68ea",
            "accessory": "Meross"
        },
        {
            "model": "MSS620",
            "name": "outdoor1",
            "deviceUrl": "192.168.1.78",
            "channel": 1,
            "messageId": "75eeaa0f086ac8e12a62420cff9eba82",
            "timestamp": 1591348163,
            "sign": "92db6bb658f43631796f6a8215b29957",
            "accessory": "Meross"
        }
    ],
    "platforms": [
        {
            "name": "Config",
            "port": 8581,
            "platform": "config"
        },
        {
            "name": "Samsung TV",
            "devices": [
                {
                    "name": "TV Salone",
                    "ip": "192.168.1.143",
                    "mac": "cc:b1:1a:ac:9a:93",
                    "inputs": [
                        {
                            "name": "raiplay",
                            "type": "app",
                            "value": "123"
                        }
                    ],
                    "switches": [
                        {
                            "name": "RaiUno",
                            "power": true,
                            "channel": 501
                        }
                    ]
                }
            ],
            "platform": "SamsungTizen"
        }
    ]
}```

**Screenshots:**

<!-- If applicable, add screenshots to help explain your problem. -->

# **Environment:**

- **Node.js Version**: <!-- node -v -->
v12.19.0
- **NPM Version**: <!-- npm -v -->
6.9.0
- **Homebridge Version**: <!-- homebridge -V -->
1.2.3
- **Homebridge Meross Plugin Version**:
homebridge-meross v3.2.0
- **Homebridge Config UI X Plugin Version**:
homebridge-config-ui-x v4.31.0
- **Operating System**: <!-- Raspbian / Ubuntu / Debian / Windows / macOS / Docker -->
Raspbian
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