Merge pull request #331 from awhitford/null-safe

Tweaked code to leverage null-safe operator.

Author: awhitford

lib/src/fields/form_builder_chips_choice.dart

@@ -148,9 +148,7 @@ class _FormBuilderChoiceChipState extends State<FormBuilderChoiceChip> {
                               FocusScope.of(context).requestFocus(FocusNode());
                               var choice = selected ? option.value : null;
                               field.didChange(choice);
-                              if (widget.onChanged != null) {
-                                widget.onChanged(choice);
-                              }
+                              widget.onChanged?.call(choice);
                             });
                           },
                   )

lib/src/fields/form_builder_chips_filter.dart

@@ -161,9 +161,7 @@ class _FormBuilderFilterChipState extends State<FormBuilderFilterChip> {
                               }
 
                               field.didChange(currentValue);
-                              if (widget.onChanged != null) {
-                                widget.onChanged(currentValue);
-                              }
+                              widget.onChanged?.call(currentValue);
                             },
                           );
                         },

lib/src/fields/form_builder_color_picker.dart

@@ -130,9 +130,7 @@ class _FormBuilderColorPickerState extends State<FormBuilderColorPicker> {
             : null);
     _textEditingController =
         TextEditingController(text: HexColor(_initialValue)?.toHex());
-    if (widget.focusNode != null) {
-      widget.focusNode.addListener(_handleFocus);
-    }
+    widget.focusNode?.addListener(_handleFocus);
     _focusNode.addListener(_handleFocus);
   }
 

lib/src/fields/form_builder_range_slider.dart

@@ -118,9 +118,7 @@ class _FormBuilderRangeSliderState extends State<FormBuilderRangeSlider> {
                       : (RangeValues values) {
                           FocusScope.of(context).requestFocus(FocusNode());
                           field.didChange(values);
-                          if (widget.onChanged != null) {
-                            widget.onChanged(values);
-                          }
+                          widget.onChanged?.call(values);
                         },
                 ),
                 Row(

lib/src/fields/form_builder_typeahead.dart

@@ -168,12 +168,9 @@ class _FormBuilderTypeAheadState<T> extends State<FormBuilderTypeAhead<T>> {
           _typeAheadController.text =
               widget.selectionToTextTransformer(suggestion);
         } else {
-          _typeAheadController.text =
-              suggestion != null ? suggestion.toString() : '';
-        }
-        if (widget.onSuggestionSelected != null) {
-          widget.onSuggestionSelected(suggestion);
+          _typeAheadController.text = suggestion?.toString() ?? '';
         }
+        widget.onSuggestionSelected?.call(suggestion);
         widget.onChanged?.call(suggestion);
       },
       getImmediateSuggestions: widget.getImmediateSuggestions,

lib/src/form_builder_custom_field.dart

@@ -63,7 +63,7 @@ class FormBuilderCustomFieldState<T> extends State<FormBuilderCustomField<T>> {
     /*return widget.formField
       ..onSaved = (T val) {
         _formState?.setValue(widget.attribute, val);
-        if (widget.formField.onSaved != null) widget.formField.onSaved(val);
+        widget.formField.onSaved?.call(val);
       }
       ..validator = (val) {
         for (int i = 0; i < widget.validators.length; i++) {
@@ -78,7 +78,7 @@ class FormBuilderCustomFieldState<T> extends State<FormBuilderCustomField<T>> {
       child: FormField(
         key: _fieldKey,
         onSaved: (val) {
-          if (widget.formField.onSaved != null) widget.formField.onSaved(val);
+          widget.formField.onSaved?.call(val);
           if (widget.valueTransformer != null) {
             var transformed = widget.valueTransformer(val);
             FormBuilder.of(context)