Fix FieldString not giving error on empty string as initial value

webview/src/FieldTypes/base.tsx

@@ -24,9 +24,9 @@ export abstract class FieldBase<T, P = {}, S = {}> extends React.Component<Props
     constructor(props: Props<T> & P) {
         super(props);
         this.state = {
-            ...this.getInitialSubState(props) as any,
             newValue: props.value,
             oldValue: props.value,
+            ...this.getInitialSubState(props) as any,
         }
     }
     public getInitialSubState(props: Props<T> & P): S {
@@ -35,6 +35,11 @@ export abstract class FieldBase<T, P = {}, S = {}> extends React.Component<Props
     public onChange = (newValue?: T) => {
         this.setState({ newValue }, () => this.props.onChange(newValue));
     }
+    public componentDidUpdate(oldProps: Props<T>) {
+        const { value } = this.props;
+        if (oldProps.value === value) return;
+        this.setState({ oldValue: value, newValue: value });
+    }
     public abstract renderInput(): React.ReactNode;
     public getError(): string | null {
         const { newValue } = this.state;

webview/src/FieldTypes/string.tsx

@@ -2,6 +2,13 @@ import * as React from 'react';
 import { FieldBase } from './base';
 
 export class FieldString extends FieldBase<string | undefined> {
+    constructor(props: FieldString['props']) {
+        super(props);
+    }
+    public getInitialSubState({ value }: FieldString['props']): FieldString['state'] {
+        value = value || undefined;
+        return { oldValue: value, newValue: value };
+    }
     public renderInput() {
         return <input value={this.state.newValue || ''} onChange={this.onChangeEvent} />;
     }