Manipulate dynamic PopupMenu drop down selector [duplicate]

Question

I am trying to get a PopupMenu in Manipulate that will be dynamically updated. The code that seems correct is below:

DynamicModule[
 {
  a = Table[
    Association[
     Table[Association[
       Table[ToString@i -> RandomInteger[50], {i, 0, 3}]], 10]], 10]
  },
 Manipulate[
  {x, y},
  {{x, First@Keys@First@a}, Keys@First@a, ControlType -> PopupMenu},
  {{y, First@Union[#[x] & /@ a]}, Union[#[x] & /@ a], 
   ControlType -> PopupMenu}
  ]
 ]

but instead I get this:

I guess I should do something like SynchronousUpdating to make it work but still have no luck