Reverted change to enum parse from string configuration value, as we can't avoid the property being evaluated before the validation has occurred.

src/Umbraco.Core/Configuration/Models/ModelsBuilderSettings.cs

@@ -30,11 +30,15 @@ namespace Umbraco.Core.Configuration.Models
         {
             get
             {
-                return Enum.TryParse<ModelsMode>(ModelsMode, true, out var value)
-                    ? value
-                    : throw new InvalidOperationException(
-                        $"Parsing of {nameof(ModelsMode)} field value of {ModelsMode} was not recognised as a valid value of the enum {nameof(ModelsMode)}. " +
-                        $"This state shouldn't have been reached as if the configuration contains an invalid valie it should be caught by {nameof(ModelsBuilderSettingsValidator)}.");
+                if (Enum.TryParse<ModelsMode>(ModelsMode, true, out var value))
+                {
+                    return value;
+                }
+
+                // We need to return somethhing valid here as this property is evalulated during start-up, and if there's an error
+                // in the configured value it won't be parsed to the enum.
+                // At run-time though this default won't be used, as an invalid value will be picked up by ModelsBuilderSettingsValidator.
+                return Configuration.ModelsMode.Nothing;
             }
         }