The setter 'observerMode' isn't defined

I got the folloing error after updagting purchases_flutter from 6.26.0 to 8.1.3. I am using Android 
studio on iMac. 

How can I fix this error

The setter 'observerMode' isn't defined for the type 'PurchasesConfiguration'. (Documentation)  Try importing the library that defines 'observerMode', correcting the name to the name of an existing setter, or defining a setter or field named 'observerMode'.

This is my code:
PurchasesConfiguration configuration;
  if (StoreConfig.isForAmazonAppstore()) {
    configuration = AmazonConfiguration(StoreConfig.instance.apiKey)
      ..appUserID = null
      ..observerMode = false;
  } else {
    configuration = PurchasesConfiguration(StoreConfig.instance.apiKey)
      ..appUserID = null
      ..observerMode = false;
  }
  await Purchases.configure(configuration);
} // end