Terraform AWS Provider Errors: "Invalid provider configuration" when using an aliased provider

In certain scenarios, when using an aliased AWS provider, you may get the following error:

│ Error: Invalid provider configuration
│ 
│ Provider "registry.terraform.io/hashicorp/aws" requires explicit configuration. Add a provider block to the root module and configure the
│ provider's required arguments as described in the provider documentation.
│ 
╵

This can be confusing, as there is an AWS provider configuration present, e.g.

provider "aws" {
    alias = "secondary"
    region = "us-east-2"
}

If you have resources that are not using the aliased provider with the provider = aws.secondary argument, they will fallback to using a provider that is not aliased, and if that provider config is missing, you will get the error mentioned above.

An example configuration where this can be exhibited is 

/* provider "aws" {
    region = "us-east-1"
}*/

provider "aws" {
    alias = "secondary"
    region = "us-east-2"
}

resource "aws_vpc" "main" {
    cidr_block = "10.0.0.0/16"
}

resource "aws_vpc" "main2" {
    provider = aws.secondary
    cidr_block = "10.0.0.0/16"
}

Running this configuration results in the above error.
Uncommenting and running the configuration again works as expected. 

This can be hard to notice in a much more complex configuration, as sometimes the provider argument in a resource is missed by mistake and the error is contradicting what is configured, which can lead to confusion.