Cannot flush updates when React is already rendering

{saveLabelFetchError && this.toggleAlertFailure(saveLabelFetchError)}

You are trying to update the dom before render cycle i.e before component has mounted. hence, you are getting an error.

Ideally, you should avoid using any library that mutates dom directly (not via react APIs) with react lib. You can read more from here

The solution is to check if there is a change in props value, if so then show error popup. And also make sure we are not mutating dom during react's render cycle.

import React, { Component } from "react";
import PropTypes from "prop-types";
import { connect } from "react-redux";
import { bindActionCreators } from "redux";

import { saveLabelValueRequest } from "../../actions/labels";

import Swal from "sweetalert2";
import "./styles.css";
import Button from "@kof/button";

export class NewLabels extends Component {
  state = {
    labelInput: "",
    prevSaveLabelFetchError: ""
  };

  static getDerivedStateFromProps(props, state) {
    if (props.saveLabelFetchError !== state.prevSaveLabelFetchError) {
      this.toggleAlertFailure(props.saveLabelFetchError);
      return {
        prevSaveLabelFetchError: props.saveLabelFetchError
      };
    }
    return null;
  }

  inputChangedhandler = e => {
    this.setState({ labelInput: e.target.value });
  };

  toggleAlertFailure = message => {
    Swal.fire({
      type: "error",
      title: "Oops...",
      text: message
    });
  };

  saveLabel = event => {
    event.persist();
    event.preventDefault();
    Swal.fire({
      title: "Are you sure?",
      text: "You won't be able to revert this!",
      type: "warning",
      showCancelButton: true,
      confirmButtonColor: "#3085d6",
      cancelButtonColor: "#d33",
      confirmButtonText: "Yes, save it."
    }).then(result => {
      if (result.value) {
        const labelKeyUuid = this.props.labelKey.uuid;
        const labels = event.target.elements.labels.value;
        this.props.saveLabelValue(labelKeyUuid, labels);
      }
    });
  };

  render() {
    const { load } = this.props;
    return (
      <div>
        <form onSubmit={this.saveLabel}>
          <textarea onChange={this.inputChangedhandler}></textarea>
          <textarea></textarea>
          <Button onClick={() => load(this.state.labelInput)}>Preview</Button>
          <Button type="submit">Save</Button>
        </form>
      </div>
    );
  }
}

NewLabels.propTypes = {
  saveLabelFetchError: PropTypes.string,
  isFetching: PropTypes.bool,
  labelKey: PropTypes.object,
  saveLabelValue: PropTypes.func
};

NewLabels.defaultProps = {
  saveLabelFetchError: "",
  labelKey: {},
  isFetching: false,
  saveLabelValue: () => {}
};

export default connect(
  state => ({
    saveLabelFetchError: state.labelsStore.saveLabelError,
    isFetching: state.labelsStore.isFetching,
    labelKey: state.labelsStore.labelKey
  }),
  dispatch =>
    bindActionCreators(
      {
        saveLabelValue: saveLabelValueRequest
      },
      dispatch
    )
)(NewLabels);

You can also look at react wrapper for sweetalert2