Null ref after switching route in React

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I suspect that your component is recreated when you go to a new route again, but the old listener is still invoked by the resize handler. Try to remove event listener in componentWillUnmount:

componentDidMount() {
    this.setCaretVis();
    window.addEventListener("resize", this.setCaretVis);
    this.setCaretVis();
}

componentWillUnmount() {
    window.removeEventListener("resize", this.setCaretVis);
}

When router recreates the component, it will subscribe to resize event again.

From the docs:

componentWillUnmount() is invoked immediately before a component is unmounted and destroyed. Perform any necessary cleanup in this method, such as invalidating timers, canceling network requests, or cleaning up any DOM elements that were created in componentDidMount