When trying to push a node to a block, the block dropdown remains empty after having chosen a frontpage and a zone.

Steps to reproduce

On eZ Publish 4.7 with eZ Flow, click on an object and select 'eZ Flow->Add to block in frontpage'. Then select a frontpage and a zone. The 'Select block'
dropdown box should be populated with a list of blocks that belong to the previously selected frontpage and zone, but it's empty.