fix _enter_scope_uncached exception when _scope is null.

Author: Oceania2018

src/TensorFlowNET.Core/Variables/variable_scope.py.cs

@@ -122,13 +122,13 @@ private VariableScope _enter_scope_uncached()
 
             if (!string.IsNullOrEmpty(_name) || _scope != null)
             {
-                var name_scope = _scope.name.Split('/').Last();
+                var name_scope = _scope == null ? _name : _scope.name.Split('/').Last();
                 if (current_name_scope == null)
                     current_name_scope = ops.name_scope(name_scope);
                 current_name_scope.__enter__();
-                var current_name_scope_name = current_name_scope;
+                string current_name_scope_name = current_name_scope;
                 _current_name_scope = current_name_scope;
-                string old_name_scope = _scope.original_name_scope;
+                string old_name_scope = _scope == null ? current_name_scope_name : _scope.original_name_scope;
 
                 if(_scope == null)
                     pure_variable_scope = new PureVariableScope(_name, old_name_scope: old_name_scope);