How to let custom property pane fields enable the apply button in SharePoint Framework

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/**
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 * @function
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 * Renders the custom field
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 */
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private render(elem: HTMLElement, ctx?, changeCallback?: (targetProperty: string, value: any) => void): void {
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  const props: ISampleInsecurePasswordHostProps = {
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    key: this.properties.key,
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    label: typeof this.label === "undefined" ? null : this.label,
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    description: typeof this.description === "undefined" ? null : this.description,
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    initialValue: typeof this.initialValue === "undefined" ? null : this.initialValue,
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    properties: this.properties,
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    targetProperty: this.targetProperty,
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    onRender: this.render,
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    onChanged: changeCallback,
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    onDispose: this.dispose,
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    onPropertyChange: this.onPropertyChange
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  };
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  // Construct the JSX properties
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  const element: React.ReactElement<ISampleInsecurePasswordProps> = React.createElement(SampleInsecurePasswordHost, props);
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  // Calls the REACT content generator
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  ReactDom.render(element, elem);
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}

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private notifyAfterValidate(oldValue: string, newValue: string) {
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  this.props.properties[this.props.targetProperty] = newValue;
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  this.props.onPropertyChange(this.props.targetProperty, oldValue, newValue);
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  // Trigger the onChanged callback
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  if (typeof this.props.onChanged !== "undefined") {
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    this.props.onChanged(this.props.targetProperty, newValue);
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  }
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}