Changed the signature of createRoutableExtension to include null

Signed-off-by: Brian Leathem <bleathem@netflix.com>

packages/core-api/src/extensions/extensions.tsx

@@ -27,8 +27,11 @@ type ComponentLoader<T> =
       sync: T;
     };
 
+// We do not use ComponentType as the return type, since it doesn't let us convey the children prop.
+// ComponentType inserts children as an optional prop whether the inner component accepts it or not,
+// making it impossible to make the usage of children type safe.
 export function createRoutableExtension<
-  T extends (props: any) => JSX.Element
+  T extends (props: any) => JSX.Element | null
 >(options: {
   component: () => Promise<T>;
   mountPoint: RouteRef;
@@ -70,6 +73,9 @@ export function createComponentExtension<
   return createReactExtension({ component });
 }
 
+// We do not use ComponentType as the return type, since it doesn't let us convey the children prop.
+// ComponentType inserts children as an optional prop whether the inner component accepts it or not,
+// making it impossible to make the usage of children type safe.
 export function createReactExtension<
   T extends (props: any) => JSX.Element | null
 >(options: {