Fix warnings

Author: mono0926

lib/src/text_input_dialog/ios_text_input_dialog.dart

@@ -19,7 +19,7 @@ class IOSTextInputDialog extends StatefulWidget {
     this.autoSubmit = false,
   });
   @override
-  _IOSTextInputDialogState createState() => _IOSTextInputDialogState();
+  State<IOSTextInputDialog> createState() => _IOSTextInputDialogState();
 
   final List<DialogTextField> textFields;
   final String? title;

lib/src/text_input_dialog/macos_text_input_dialog.dart

@@ -22,7 +22,7 @@ class MacOSTextInputDialog extends StatefulWidget {
     this.autoSubmit = false,
   });
   @override
-  _MacOSTextInputDialogState createState() => _MacOSTextInputDialogState();
+  State<MacOSTextInputDialog> createState() => _MacOSTextInputDialogState();
 
   final List<DialogTextField> textFields;
   final String? title;

lib/src/text_input_dialog/material_text_input_dialog.dart

@@ -20,7 +20,7 @@ class MaterialTextInputDialog extends StatefulWidget {
     this.autoSubmit = false,
   });
   @override
-  _MaterialTextInputDialogState createState() =>
+  State<MaterialTextInputDialog> createState() =>
       _MaterialTextInputDialogState();
 
   final List<DialogTextField> textFields;