Fixed error 'The getter 'initialValue' was called on null' if no FormBuiderState ancestry. Closes #364

Author: danvick

lib/src/fields/form_builder_dropdown.dart

@@ -83,8 +83,8 @@ class _FormBuilderDropdownState<T> extends State<FormBuilderDropdown<T>> {
     _formState = FormBuilder.of(context);
     _formState?.registerFieldKey(widget.attribute, _fieldKey);
     _initialValue = widget.initialValue ??
-        (_formState.initialValue.containsKey(widget.attribute)
-            ? _formState.initialValue[widget.attribute]
+        ((_formState?.initialValue?.containsKey(widget.attribute) ?? false)
+            ? _formState?.initialValue[widget.attribute]
             : null);
     super.initState();
   }