SSR support: Don't import popper if window undefined

[winton]

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Changes in this pull request

• Checking that window is defined before requiring and exporting popper-mixins

[fatlinesofcode]

Hi @winton I've tried to add this fix to my nextjs project but am still receiving error popper window error

{ ReferenceError: window is not defined
    at /Users/phil/Sites/git/clients/brandcrush/front/node_modules/element-react-ssr/dist/npm/es5/libs/utils/popper.js:57:16
    at root (/Users/phil/Sites/git/clients/brandcrush/front/node_modules/element-react-ssr/dist/npm/es5/libs/utils/popper.js:48:26)
    at Object.<anonymous> (/Users/phil/Sites/git/clients/brandcrush/front/node_modules/element-react-ssr/dist/npm/es5/libs/utils/popper.js:53:3)

Have you attempted to use this with nextjs?

[arthurhamon]

Can someone work on this fix ?