fix: custom field numbers

Nil check when collecting the existing automatically
named custom field numbers.

#1041

Author: pvaiko

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