#3483496 - Made autoselect for a single option in the select. Simplified the...

#3483496 - Made autoselect for a single option in the select. Simplified the wording in Referenced entities.

Closes #3483496

src/Plugin/UiPatterns/Source/DerivableContextSourceBase.php

@@ -278,6 +278,10 @@ abstract class DerivableContextSourceBase extends SourcePluginBase {
         }
       }
     }
+    // Set default value to the only available option.
+    if (!$derivable_context && count($options_derivable_contexts) === 1) {
+      $derivable_context = array_key_first($options_derivable_contexts);
+    }
     return $derivable_context;
   }