'Selected RTP Stream Configuration': characteristic was supplied illegal value: null!
First of all, I would like to thank you for creating this fantastic plugin. I have been extremely happy with it for the 8–9 months I have been running it so far. Your effort on this project is greatly appreciated!

**Describe The Problem:**
I recently updated to the latest version (`v4.3.5`) and, unfortunately, I could not access my camera feeds from the Apple Home app after the update. I have three G3 Flex cameras set up on my network. As you can see in the logs below, it seems as if the RTP stream configuration is not set up correctly. I'm unsure of how to resolve this, as I don't recall having to configure any RTP-related settings when I first installed the plugin.

For your reference, I am running the latest version of Homebridge (`v1.3.3`) and Homebridge Config UI X (`v4.40.0`).

**To Reproduce:**
Install the latest version of the `homebridge-unifi-protect` plugin and configure it with one or more G3 Flex cameras, following the installation guide on the front page of this GitHub repo. The error displayed in the logs below will show up.

**Logs:**
```
[3/10/2021, 8:46:56 AM] [homebridge-unifi-protect] This plugin generated a warning from the characteristic 'Selected RTP Stream Configuration': characteristic was supplied illegal value: null! Home App will reject null for Apple defined characteristics. See https://git.io/JtMGR for more info.
```

**Homebridge Configuration:**
```json
{
    "bridge": {
        "name": "Homebridge",
        "username": "CC:22:3D:E3:CE:30",
        "manufacturer": "homebridge.io",
        "model": "homebridge",
        "port": 51826,
        "pin": "********"
    },
    "ports": {
        "start": 52100,
        "end": 52150
    },
    "platforms": [
        {
            "name": "Config",
            "port": 8581,
            "auth": "none",
            "theme": "auto",
            "tempUnits": "c",
            "lang": "auto",
            "sudo": false,
            "log": {
                "method": "systemd"
            },
            "platform": "config"
        },
        {
            "controllers": [
                {
                    "address": "192.168.1.7",
                    "password": "********",
                    "username": "homebridge"
                }
            ],
            "options": [
                "Enable.Stream.High"
            ],
            "platform": "UniFi Protect"
        }
    ],
    "accessories": []
}
```

**Screenshots:**
![image](https://user-images.githubusercontent.com/660576/110595856-e0a98300-817e-11eb-8a75-6586824fc22d.png)

**Environment:**
* **Homebridge Version:** 1.3.3
* **Node Version:** 12.18.4
* **Homebridge-UniFi-Protect Plugin Version:** 4.3.5
* **UniFi Protect Version:** 1.13.7
* **UniFi Protect Controller Type:** Unifi Cloud Key Gen2 Plus
* **UniFi Protect Controller Firmware Version:** 1.1.13
* **Apple Device and iOS / macOS / iPadOS / tvOS Version:** Apple TV 4K 64GB (most recent version)
* **Operating System and OS Version:** Debian
