Unsupported operation: Platform._operatingSystem

You can use a try-catch block to prevent the exception from breaking the flow:

bool kisweb;
try{
    if(Platform.isAndroid||Platform.isIOS) {
        kisweb=false;
    } else {
        kisweb=true;
    }
} catch(e){
    kisweb=true;
}

I reopen this issue to give a more appropriate answer which is now available in native in flutter:

import 'package:flutter/foundation.dart';
if (defaultTargetPlatform == TargetPlatform.iOS || defaultTargetPlatform == TargetPlatform.android) {
    // Some android/ios specific code
}
else if (defaultTargetPlatform == TargetPlatform.linux || defaultTargetPlatform == TargetPlatform.macOS || defaultTargetPlatform == TargetPlatform.windows) {
    // Some desktop specific code there
}
else {
    // Some web specific code there
}