introduced temp cache for form fields that get removed from memory

Author: ahmednfwela

packages/flutter_form_builder/lib/src/fields/form_builder_date_time_picker.dart

@@ -260,7 +260,8 @@ class _FormBuilderDateTimePickerState
     super.initState();
     _textFieldController = widget.controller ?? TextEditingController();
     _dateFormat = widget.format ?? _getDefaultDateTimeFormat();
-    final initVal = initialValue;
+    //setting this to value instead of initialValue here is OK since we handle initial value in the parent class
+    final initVal = value;
     _textFieldController.text =
         initVal == null ? '' : _dateFormat.format(initVal);
     effectiveFocusNode.addListener(_handleFocus);

packages/flutter_form_builder/lib/src/fields/form_builder_text_field.dart

@@ -437,8 +437,8 @@ class _FormBuilderTextFieldState
   @override
   void initState() {
     super.initState();
-    _controller =
-        widget.controller ?? TextEditingController(text: initialValue);
+    //setting this to value instead of initialValue here is OK since we handle initial value in the parent class
+    _controller = widget.controller ?? TextEditingController(text: value);
     _controller!.addListener(_handleControllerChanged);
   }
 

packages/flutter_form_builder/lib/src/form_builder.dart

@@ -122,19 +122,29 @@ class FormBuilderState extends State<FormBuilder> {
     if (oldField != null) {
       // ignore: invalid_use_of_protected_member
       field.setValue(oldField.value);
+      _tempFieldValues.remove(name);
     } else {
-      // ignore: invalid_use_of_protected_member
-      field.setValue(field.initialValue);
+      final oldTemp = _tempFieldValues[name];
+      if (oldTemp != null) {
+        // ignore: invalid_use_of_protected_member
+        field.setValue(oldTemp);
+        _tempFieldValues.remove(name);
+      } else {
+        // ignore: invalid_use_of_protected_member
+        field.setValue(field.initialValue);
+      }
     }
   }
 
+  final _tempFieldValues = <String, dynamic>{};
   void unregisterField(String name, FormBuilderFieldState field) {
     assert(_fields.containsKey(name));
     // Only remove the field when it is the one registered.  It's possible that
     // the field is replaced (registerField is called twice for a given name)
     // before unregisterField is called for the name, so just emit a warning
     // since it may be intentional.
     if (field == _fields[name]) {
+      _tempFieldValues[name] = field.value;
       _fields.remove(name);
     } else {
       assert(() {