UiWebsocket.py: "'SiteStorage' object has no attribute 'piecefields'"

### Step 1: Please describe your environment

* ZeroNet version: ZeroNet-py3 0.7.2 (rev 4553)
* Operating system: Debian Buster
* Web browser: Google Chrome
* Tor status: always
* Opened port: no
* Special configuration: UiPassword is on, access through local network.

### Step 2: Describe the problem:

#### Steps to reproduce:

1. Open link http://:43110/149EPBN4hQRpwj5TcoPETepnoXHCUXzaCN/ (YouTube on ZeroNet)
2. It suddenly appears a message "Internal error: AttributeError("'SiteStorage' object has no attribute 'piecefields'"): 'SiteStorage' object has no attribute 'piecefields' >> UiWebsocket.py line 80 > 236 > OptionalManager/UiWebsocketPlugin.py line 169 > 67" on top right corner, and the zite does not show the video list.

#### Observed Results:


#### Expected Results:

Being able to use YouTube on ZeroNet again.

Start with UiWebsocket.py around line 80 and OptionalManager/UiWebsocketPlugin.py around line 169, then trace how SiteStorage is used during the reported YouTube zite request. Reproduce the error using the listed ZeroNet version and steps. Done means the zite loads its video list without the SiteStorage piecefields AttributeError.

索引モデルが issue の本文から書いたものです。