Merge pull request #221 from Freddy03h/dom-api-null

getElementById and getSelection could return null

Author: nojaf

src/DOMAPI/Document.res

@@ -15,7 +15,7 @@ Returns the first element within node's descendants whose ID is elementId.
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/getElementById)
 */
 @send
-external getElementById: (document, string) => element = "getElementById"
+external getElementById: (document, string) => null<element> = "getElementById"
 
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/getAnimations)
@@ -443,7 +443,7 @@ Returns an object representing the current selection of the document that is loa
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/getSelection)
 */
 @send
-external getSelection: document => selection = "getSelection"
+external getSelection: document => null<selection> = "getSelection"
 
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Document/hasStorageAccess)

src/DOMAPI/DocumentFragment.res

@@ -38,7 +38,7 @@ Returns the first element within node's descendants whose ID is elementId.
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/DocumentFragment/getElementById)
 */
   @send
-  external getElementById: (T.t, string) => element = "getElementById"
+  external getElementById: (T.t, string) => null<element> = "getElementById"
 
   /**
 Inserts nodes before the first child of node, while replacing strings in nodes with equivalent Text nodes.

src/DOMAPI/Window.res

@@ -514,4 +514,4 @@ external cancelIdleCallback: (window, int) => unit = "cancelIdleCallback"
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Window/getSelection)
 */
 @send
-external getSelection: window => selection = "getSelection"
+external getSelection: window => null<selection> = "getSelection"

src/Global.res

@@ -744,4 +744,4 @@ external cancelIdleCallback: int => unit = "cancelIdleCallback"
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Window/getSelection)
 */
-external getSelection: unit => selection = "getSelection"
+external getSelection: unit => null<selection> = "getSelection"