Merge pull request #1067 from Courseplay/1041-unable-to-create-new-custom-fields-fs25-v11101

fix: custom field numbers

Author: Tensuko

scripts/field/CustomFieldManager.lua

@@ -72,7 +72,9 @@ function CustomFieldManager:getNewFieldNumber()
         local name = entry:getName()
         if name:startsWith("CP-") then 
             local n = entry:getFieldNumber()
-            numbers[n] = true
+            if n then
+                numbers[n] = true
+            end
         end
     end
     local ix = 1