Cannot read property of undefined in react-dom

Forge Forge UI Kit and Custom UI
,
1

Hello,
I am working on a content Action opening a modal. Everything works fine, but when the modal is closed I get the following error in the console:

react-dom.profiling.min.js:205 TypeError: Cannot read properties of undefined (reading 'type')
    at bn (RendererNextComponent.tsx:204:50)

which in turn links to

const isFirstChildAForm = forgeDoc.children[0].type === 'Form';

This prevents the action menu from opening again, i.e. if you click on it, it opens and immediately closes.
Any idea?

		<Modal onClose={close}>
				<Form onSubmit={handleSubmit(submit)}>
					<ModalHeader title="Create Page">
						<ModalTitle>Create Page</ModalTitle>
					</ModalHeader>
					<ModalBody>
						<Text>Create Pagee</Text>
						<Label>
							Select
							<RequiredAsterisk />
						</Label>
						<Select
							isRequired="true"
							name="page"
							options={pageListSelect ? pageListSelect : "loading..."}
							{...register("select")}
						/>
						<Label>
							Name of the new page
							<RequiredAsterisk />
						</Label>
						<Textfield
							{...register("pagename", {
								required: true,
								defaultValue: "New Page",
							})}
						/>
					</ModalBody>
					<ModalFooter>
						<Text>
							Create a new child Page
						</Text>
						<Button onClick={close} appearance="subtle" type="button">
							Cancel
						</Button>
						<Button appearance="primary" type="submit">
							Submit
						</Button>
					</ModalFooter>
				</Form>
			</Modal>