panic: on invalid depends_on reference root

Terraform Version

Terraform v0.12.0

Terraform Configuration Files

resource "null_resource" "one" {
}

resource "null_resource" "two" {
  depends_on = ["crash"]
}

Debug Output

https://gist.github.com/tinocode/74f4d413fb04b20b20055685574f6da3

Crash Output

https://gist.github.com/tinocode/56d0916eabdd77bad638cf22f81923a0

Expected Behavior

Terraform reports an error about the unknown reference:

unix% ~/Downloads/terraform-0.11.14 plan

Error: null_resource.two: resource depends on non-existent resource 'crash'

Actual Behavior

Terraform crashed in a Go panic.

Steps to Reproduce

cat > main.tf <<EOF
resource "null_resource" "one" {
}

resource "null_resource" "two" {
  depends_on = ["crash"]
}
EOF

terraform init
terraform plan

Additional Context

This actually came to light when I mistyped a depends_on reference as "null_resourceone" instead of "null_resource.one".

References

#21589

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