null checks

* from Omega-CMS fork on this code
https://github.com/Omega-CMS/omega-gridstack.js

Author: adumesny

src/gridstack-dd.ts

@@ -123,7 +123,7 @@ GridStack.prototype._setupAcceptWidget = function(): GridStack {
 
       // see if we already have a node with widget/height and check for attributes
       let node = el.gridstackNode || {};
-      if (!node.width || !node.height) {
+      if (el.getAttribute && (!node.width || !node.height)) {
         let w = parseInt(el.getAttribute('data-gs-width'));
         if (w > 0) { node.width = w; }
         let h = parseInt(el.getAttribute('data-gs-height'));
@@ -180,6 +180,7 @@ GridStack.prototype._setupAcceptWidget = function(): GridStack {
       }
 
       let node = el.gridstackNode; // use existing placeholder node as it's already in our list with drop location
+      if (!node) { return false; }
       const _id = node._id;
       this.engine.cleanupNode(node); // removes all internal _xyz values (including the _id so add that back)
       node._id = _id;

src/gridstack.ts

@@ -1129,7 +1129,7 @@ export class GridStack {
     // check for css min height
     let cssMinHeight = parseInt(getComputedStyle(this.el)['min-height']);
     if (cssMinHeight > 0) {
-      let minRow =  Math.round(cssMinHeight / this.getCellHeight(true));
+      let minRow = Math.round(cssMinHeight / this.getCellHeight(true));
       if (row < minRow) {
         row = minRow;
       }

src/utils.ts

@@ -76,7 +76,7 @@ export class Utils {
       if (els[0] === '#') {
         return document.getElementById(els.substring(1));
       }
-      if (els[0] === '.') {
+      if (els[0] === '.' || els[0] === '[') {
         return document.querySelector(els);
       }