fix(getPrefix): ensure documentElement.style actually exists

Fixes #574, #575

lib/utils/getPrefix.js

@@ -1,12 +1,14 @@
 // @flow
 const prefixes = ['Moz', 'Webkit', 'O', 'ms'];
 export function getPrefix(prop: string='transform'): string {
-  // Checking specifically for 'window.document' is for pseudo-browser server-side
-  // environments that define 'window' as the global context.
-  // E.g. React-rails (see https://github.com/reactjs/react-rails/pull/84)
-  if (typeof window === 'undefined' || typeof window.document === 'undefined') return '';
-
-  const style = window.document.documentElement.style;
+  // Ensure we're running in an environment where there is actually a global
+  // `window` obj
+  if (typeof window === 'undefined') return '';
+
+  // If we're in a pseudo-browser server-side environment, this access
+  // path may not exist, so bail out if it doesn't.
+  const style = window.document?.documentElement?.style;
+  if (!style) return '';
 
   if (prop in style) return '';