[WordPress Gutenberg] Check next block type in block editor.

block.js

const  { registerBlockType, registerBlockStyle } = wp.blocks;
const { select } = wp.data;

/**
 * Get next blokc from client ID.
 */
const getNextBlock = ( id ) => {
	const allBlocks = select( 'core/block-editor' ).getBlocks();
	let index = null;
	for ( let i = 0; i < allBlocks.length; i++ ) {
		if ( allBlocks[i].clientId === id ) {
			index = i;
			break;
		}
	}
	if ( null === index ) {
		return false;
	}
	return allBlocks[ index + 1 ] || false;
}

registerBlockType( 'my-blocks/block', {

	// clientId is the ID of this block.
	edit( { attributes, clientId, setAttributes } ) {
		const nextBlock = getNextBlock( clientId );
		
		return (
			<div>
				<p>This block must precedes page break.</p>
				{ ( nextBlock && 'core/nextpage' === nextBlock.name  ) && (
					<p class="warning">Hey, put pabebreak after this!!</p>
				) }
			</div>
		);
	}

} );