Prevent SPFx from marking React as an external so that React in included in your bundle

SPFxKeepReactGulpFile.js

const gulp = require('gulp');
const build = require('@microsoft/sp-build-web');
build.addSuppression("Warning - [sass] The local CSS class 'ms-Grid' is not camelCase and will not be type-safe.");

build.configureWebpack.mergeConfig({
    additionalConfiguration: (generatedConfiguration) => {
        generatedConfiguration.externals = generatedConfiguration.externals
            .filter(name => !(["react", "react-dom"].includes(name)))
        return generatedConfiguration;
    }
});

build.initialize(gulp);