invalid value for input field for additional production makes it disappear

[britiscurious]

• enable "Show input field for additional production (negative values possible)"
• enter 100 for Farmers
• delete the value from the input field for additional production of the Fishery tile

observed behavior:
the Fishery tile disappears

expected result:
the value is not accepted, the old value is restored, but foremost the tile does not disappear

[britiscurious]

see my pull request for a fix