force typescript 3.6 as 3.7 has breaking change

* force Typescript 3.6.5 as >3.7 has getter breaking change forcing all
users of this lib to be >3.7 to compile.
* they should have made this a 4.x thing
see microsoft/TypeScript#33939
otherwise you get:
error TS1086: An accessor cannot be declared in an ambient context.

Author: adumesny

package.json

@@ -54,7 +54,7 @@
     "core-js": "^3.6.4",
     "coveralls": "^3.0.9",
     "doctoc": "^1.4.0",
-    "eslint": "^7.12.1",
+    "eslint": "^7.14",
     "grunt": "^1.0.4",
     "grunt-cli": "^1.3.2",
     "grunt-contrib-connect": "^3.0.0",
@@ -67,16 +67,17 @@
     "grunt-protractor-webdriver": "^0.2.5",
     "grunt-sass": "3.1.0",
     "jasmine-core": "^3.5.0",
-    "karma": "^5.2.3",
+    "karma": "^4",
     "karma-chrome-launcher": "^3.1.0",
     "karma-cli": "^2.0.0",
     "karma-jasmine": "^4.0.1",
     "karma-typescript": "4.1.1",
     "node-sass": "^5.0.0",
+    "protractor": "^7.0.0",
     "puppeteer": "^5.4.1",
     "serve-static": "^1.14.1",
     "ts-loader": "^8.0.7",
-    "typescript": "^4.0.5",
+    "typescript": "3.6.5",
     "webpack": "^5.3.2",
     "webpack-cli": "^4.1.0"
   }

src/h5/dd-draggable.ts

@@ -246,7 +246,8 @@ export class DDDraggable extends DDBaseImplement implements HTMLElementExtendOpt
   /** @internal */
   private _removeHelperStyle(): DDDraggable {
     // don't bother restoring styles if we're gonna remove anyway...
-    if (! (this.helper as GridItemHTMLElement)?.gridstackNode?._isAboutToRemove) {
+    let node = this.helper ? (this.helper as GridItemHTMLElement).gridstackNode : undefined;
+    if (!node || !node._isAboutToRemove) {
       DDDraggable.originStyleProp.forEach(prop => {
         this.helper.style[prop] = this.dragElementOriginStyle[prop] || null;
       });