[SalesForce] Onchange Javascript to update input field on VF page When an item is selected on Selected list

With the new Winter 17 release existing custom picklist fields can now be marked as required at a field level.

To create or modify custom picklists:

Picklist on an object — Use the fields area of the object.

Global picklist — From Setup, enter picklist in the Quick Find box, then select > Picklists.

Taken from the release notes for Winter 17.

This appears to work depending on which objects are selected in the first list. My guess at this point is that there is some permissions issue where my user doesn't have access to the fields on all objects when accessed via -

Map<String,Schema.SObjectType> gd = Schema.getGlobalDescribe();  
Schema.SObjectType sobjType = gd.get(myobj.ObjectName);  

Updated with full solution:

I added a filter to my code that builds the Object list in the first dropdown. For each object type I try to describe it and if that returns null I don't add it as an option. Now whatever object I select the field dropdown is dynamically updated with the associated fields.

I think this solution will work for me. There may be a better answer to select the filtered list of objects using the describe methods?

public List<SelectOption> getSobjectFields()   
{  
    List<SelectOption> fields = new List<SelectOption>(); 

    Schema.SObjectType sobjType = gd.get(myobj.ObjectName);  
    if (sobjType != null) {
    Schema.DescribeSObjectResult r = sobjType.getDescribe();  
    Map<String,Schema.SObjectField> M = r.fields.getMap();  

    for(String fieldName : M.keyset())  
        {  
            Schema.SObjectField field = M.get(fieldName);                                                      
            Schema.DescribeFieldResult fieldDesc = field.getDescribe();  
            fields.add(new SelectOption(fieldName.toLowerCase(),fieldName.toLowerCase())); 
        }  
    }


    return fields;  
}      

public List<SelectOption> getObjectNames()
{
   List<Schema.SObjectType> gdo = Schema.getGlobalDescribe().Values(); 
   List<SelectOption> options = new List<SelectOption>();

   for(Schema.SObjectType f : gdo)
   {
       Schema.SObjectType sobjType = gd.get(f.getDescribe().getLabel());  
       if (sobjType != null) {
          options.add(new        SelectOption(f.getDescribe().getLabel(),f.getDescribe().getLabel()));
       }
   }
   return options;

}