Satisfy strict null checks

src/helpers/attachto.ts

@@ -16,7 +16,9 @@ function post(_: any, vnode: VNode): void {
 
 function destroy(vnode: VNode): void {
   // Remove placeholder
-  vnode.elm && vnode.elm.parentElement.removeChild(vnode.elm);
+  if (vnode.elm !== undefined) {
+    (vnode.elm.parentElement as HTMLElement).removeChild(vnode.elm);
+  }
   // Remove real element from where it was inserted
   vnode.elm = (vnode.data as VNodeData).attachData.real;
 }

src/htmldomapi.ts

@@ -35,11 +35,11 @@ function appendChild(node: Node, child: Node): void {
   node.appendChild(child);
 }
 
-function parentNode(node: Node): HTMLElement {
+function parentNode(node: Node): HTMLElement | null {
   return node.parentElement;
 }
 
-function nextSibling(node: Node): Node {
+function nextSibling(node: Node): Node | null {
   return node.nextSibling;
 }
 
@@ -63,4 +63,5 @@ export const htmlDomApi = {
   tagName,
   setTextContent,
 } as DOMAPI;
+
 export default htmlDomApi;