selectChannel: sync auto-resolved value during mounting.

## Issue
Fixes `4621 Can't create Comments if you recently deleted a channel`

The `channel` that the parent passes in is from a persisted state. If the channel has been deleted, `<select>` will automatically resolve to another selectable value. However, `onChange` will not be called for this scenario, so we now have a mismatch.

## Changes
- Manually check if the value has been auto-resolved and report it to the parent.
- An extra `setTimeout` was needed. It seems like `onChannelChange` needs to be called after the first `useEffect` of the parent, otherwise the call has no effect.

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Fixed
 - Fix sluggish Back button when navigation back to channels with lots of comments _community pr!_ ([#4576](https://github.com/lbryio/lbry-desktop/pull/4576))
 - Fix 'Related' and 'Comments' section lazy-load not working in some scenarios _community pr!_ ([#4586](https://github.com/lbryio/lbry-desktop/pull/4586))
+- Fix comment-creation failure if you have recently deleted a channel _community pr!_ ([#4630](https://github.com/lbryio/lbry-desktop/pull/4630))
 
 ## [0.47.1] - [2020-07-23]
 

ui/component/selectChannel/view.jsx

@@ -23,6 +23,8 @@ type State = {
   addingChannel: boolean,
 };
 
+const ID_FF_SELECT_CHANNEL = 'ID_FF_SELECT_CHANNEL';
+
 class ChannelSelection extends React.PureComponent<Props, State> {
   constructor(props: Props) {
     super(props);
@@ -36,7 +38,7 @@ class ChannelSelection extends React.PureComponent<Props, State> {
   }
 
   componentDidMount() {
-    const { channels, fetchChannelListMine, fetchingChannels, emailVerified } = this.props;
+    const { channel, channels, fetchChannelListMine, fetchingChannels, emailVerified, onChannelChange } = this.props;
     if (IS_WEB && !emailVerified) {
       return;
     }
@@ -44,6 +46,15 @@ class ChannelSelection extends React.PureComponent<Props, State> {
     if ((!channels || !channels.length) && !fetchingChannels) {
       fetchChannelListMine();
     }
+
+    const elem = document.getElementById(ID_FF_SELECT_CHANNEL);
+    // $FlowFixMe
+    if (elem && elem.value && elem.value !== channel) {
+      setTimeout(() => {
+        // $FlowFixMe
+        onChannelChange(elem.value);
+      }, 250);
+    }
   }
 
   componentDidUpdate() {
@@ -84,6 +95,7 @@ class ChannelSelection extends React.PureComponent<Props, State> {
     return (
       <Fragment>
         <FormField
+          id={ID_FF_SELECT_CHANNEL}
           name="channel"
           label={label || __('Channel')}
           type="select"