fix(core): correct isDef typeguard

Signed-off-by: braks <78412429+bcakmakoglu@users.noreply.github.com>

packages/core/src/utils/store.ts

@@ -1,7 +1,11 @@
 import type { Actions, Connection, Edge, GraphEdge, GraphNode, Node, State } from '~/types'
 
-export function isDef<T>(val: T): val is NonNullable<T> {
+type NonUndefined<T> = T extends undefined ? never : T
-  return typeof unref(val) !== 'undefined'
+
+export function isDef<T>(val: T): val is NonUndefined<T> {
+  const unrefVal = unref(val)
+
+  return typeof unrefVal !== 'undefined' && unrefVal !== null
 }
 
 export function addEdgeToStore(edgeParams: Edge | Connection, edges: Edge[], onError: State['hooks']['error']['trigger']) {