Hi bekir205,
Welcome to Apple Support Communities. Since it sounds like you recently restored this iPhone from a backup, it may just need more time to complete. As mentioned in Restore your iPhone, iPad, or iPod touch from a backup, "Content like your apps, photos, music, and other information will continue to restore in the background for the next several hours or days, depending on the amount of information there is."
If the same continues after that time and only impacts WhatsApp, you can try the app specific troubleshooting from If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open. Guidance is as follows.
1 Close and reopen the app
Force the app to close. Then open the app to see if it works as expected.
2 Restart your device
Restart your iPhone or restart your iPad. Then open the app to see if it works as expected.
3 Check for updates
To see if updates are available for the app, open the App Store, tap your profile icon, then scroll down to see if updates are available. If updates aren't available for the app, update the software on your iPhone or iPad.
4 Delete the app, then redownload it
Delete the app. When you do this, you might lose data that was stored in the app. Then redownload the app. Then check to see if the problem with the app is fixed.
Take care.