fix(ssr): added early return on init & initAll when document is undefined

Author: stephenjason89

src/gridstack.ts

@@ -94,6 +94,9 @@ export class GridStack {
    * let grid = document.querySelector('.grid-stack').gridstack;
    */
   public static init(options: GridStackOptions = {}, elOrString: GridStackElement = '.grid-stack'): GridStack {
+    if(typeof document === 'undefined') {
+      return null
+    }
     let el = GridStack.getGridElement(elOrString);
     if (!el) {
       if (typeof elOrString === 'string') {
@@ -121,6 +124,9 @@ export class GridStack {
    */
   public static initAll(options: GridStackOptions = {}, selector = '.grid-stack'): GridStack[] {
     let grids: GridStack[] = [];
+    if(typeof document === 'undefined') {
+      return grids
+    }
     GridStack.getGridElements(selector).forEach(el => {
       if (!el.gridstack) {
         el.gridstack = new GridStack(el, Utils.cloneDeep(options));