Small bug fixes - #202

'RoomsPage.PropertyChanged' hides inherited member 'AvaloniaObject.PropertyChanged'. Use the new keyword if hiding was intended.

'RoomDetailsPage.PropertyChanged' hides inherited member 'AvaloniaObject.PropertyChanged'. Use the new keyword if hiding was intended.

'PopupWindow.PropertyChanged' hides inherited member 'AvaloniaObject.PropertyChanged'. Use the new keyword if hiding was intended.

'FriendsPage.PropertyChanged' hides inherited member 'AvaloniaObject.PropertyChanged'. Use the new keyword if hiding was intended.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Nullability of reference types in type of parameter 'parameter' of 'object NullToBoolConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)' doesn't match implicitly implemented member 'object? IValueConverter.ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)' (possibly because of nullability attributes).

Nullability of reference types in type of parameter 'value' of 'object NullToBoolConverter.ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)' doesn't match implicitly implemented member 'object? IValueConverter.ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)' (possibly because of nullability attributes).