There is a bug in Store.java (GXT 3.0-beta1)

PHP Code:
public boolean isCurrentValue(M model) {
    return (
access.getValue(model) == null && value == null) || access.getValue(model).equals(value);

If previous value was null and new value is not null, then function throws NullPointerException.
Because access.getValue(model) returns null reference.

I'm using GridRowEditing for editing my grid.

PHP Code:
final GridEditing<AuthorProxyediting = new GridRowEditing<AuthorProxy>(grid);
editing.addEditor(cc1, new TextField()); 
Workaround

Set fields (if possible) to non-null values before inserting to store.

PHP Code:
final AuthorProxy authorMutable authorRequest.edit(author);
authorMutable.setNameFirst("");
authorMutable.setNameLast("");