Hi Jati
The problem is almost certainly your Avast virus checker. Avast use a hueuristic technique to detect virus and trojen malware . See these post please Redirecting to Google Groups
The problem is Avast is a third rate virus checker and is very sensitive and creates 'false positive' identifications of App Inventor apps sometimes even though there is NO trojan/virus.
You might check your apk on virustotal.com You can see what it says about your app checked on various other virus programs
Most probably, only Avast is saying you have a virus. You can complain to Avast or you can load a different free checker.