Fixes #2666 - see comments in the code to explain how.

Author: Dunbaratu

src/kOS.Safe/Exceptions/KOSLookupFailException.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 
 namespace kOS.Safe.Exceptions
 {
@@ -30,7 +30,7 @@ public class KOSLookupFailException: KOSException
         /// <param name="exist">True if the reason for the message is that the thing exists at
         ///   other times, but is just not available at the moment.</param>
         public KOSLookupFailException(string category, string lookupName, object obj, bool exist = false) :
-            base(String.Format( (exist ? TERSE_NOT_EXIST_MSG_FMT : TERSE_NOT_EXIST_MSG_FMT), category, lookupName.ToUpper(), obj.ToString()) )
+            base(String.Format( (exist ? TERSE_DOES_EXIST_MSG_FMT : TERSE_NOT_EXIST_MSG_FMT), category, lookupName.ToUpper(), obj.ToString()) )
         {
             this.category = category;
             this.lookupName = lookupName;

src/kOS/Suffixed/PartModuleField/PartModuleFields.cs

@@ -235,8 +235,23 @@ public virtual BooleanValue HasField(StringValue fieldName)
         /// <returns>a BaseField - a KSP type that can be used to get the value, or its GUI name or its reflection info.</returns>
         protected BaseField GetField(string cookedGuiName)
         {
-            return partModule.Fields.Cast<BaseField>().
-                FirstOrDefault(field => string.Equals(GetFieldName(field), cookedGuiName, StringComparison.CurrentCultureIgnoreCase));
+            // Conceptually this should be a FirstOrDefault(), because there should only be one Field
+            // with the given GUI name.  But Issue #2666 forced kOS to change it to a list of hits
+            // because KSP started naming two fields with the same gui name, only one of which is visible
+            // at a time:
+            IEnumerable<BaseField> allMatches = partModule.Fields.Cast<BaseField>().
+                Where(field => string.Equals(GetFieldName(field), cookedGuiName, StringComparison.CurrentCultureIgnoreCase));
+            // When KSP is *not* doing the weird thing of two fields with the same name, there's just one hit and it's simple:
+            if (allMatches.Count() == 1)
+                return allMatches.First();
+            if (allMatches.Count() == 0)
+                return null;
+
+            // Issue #2666 is handled here.  kOS should not return the invisible field when there's
+            // a visible one of the same name it could have picked instead.  Only return an invisible
+            // field if there's no visible one to pick.
+            BaseField preferredMatch = allMatches.FirstOrDefault(field => FieldIsVisible(field));
+            return preferredMatch ?? allMatches.First();
         }
 
         /// <summary>