[Fix] searchDecoration parameter was null

lib/country_code_picker.dart

@@ -25,7 +25,7 @@ class CountryCodePicker extends StatefulWidget {
     this.textStyle,
     this.padding = const EdgeInsets.all(0.0),
     this.showCountryOnly = false,
-    this.searchDecoration,
+    this.searchDecoration = const InputDecoration(),
     this.searchStyle,
     this.emptySearchBuilder,
   });

lib/selection_dialog.dart

@@ -19,6 +19,7 @@ class SelectionDialog extends StatefulWidget {
     InputDecoration searchDecoration = const InputDecoration(),
     this.searchStyle,
   }) :
+    assert(searchDecoration != null, 'searchDecoration must not be null!'),
     this.searchDecoration = searchDecoration.copyWith(prefixIcon: Icon(Icons.search)),
     super(key: key);