How to fix Error: Null value found in list in Terraform

# Instead of this (may contain nulls):
variable "tags" {
  default = [var.optional_tag, "production"]
}

# Use this:
locals {
  tags = compact([var.optional_tag, "production"])
}

resource "aws_instance" "example" {
  tags = local.tags
}

# For any list type, filter out nulls:
locals {
  security_group_ids = [for sg_id in var.optional_security_groups : sg_id if sg_id != null]
}

resource "aws_instance" "example" {
  vpc_security_group_ids = local.security_group_ids
}

# Or filter directly in the resource argument:
resource "aws_instance" "example" {
  vpc_security_group_ids = [for sg_id in var.sg_list : sg_id if sg_id != null]
}

# Instead of:
variable "optional_protocol" {
  default = null
}

resource "aws_security_group_rule" "example" {
  from_port   = 443
  to_port     = 443
  protocol    = var.optional_protocol  # This could be null!
}

# Use coalesce to provide a default:
resource "aws_security_group_rule" "example" {
  from_port   = 443
  to_port     = 443
  protocol    = coalesce(var.optional_protocol, "tcp")
}

# Instead of:
data "aws_ami" "ubuntu" {
  filter {
    name   = "name"
    values = [var.optional_ami_name]  # Might be null!
  }
}

# Use try() with a fallback:
locals {
  # Returns [] if the lookup fails or returns null
  ami_list = try([data.aws_ami.ubuntu.id], [])
}

resource "aws_instance" "example" {
  ami = try(data.aws_ami.ubuntu.id, var.fallback_ami_id)
}

variable "additional_security_groups" {
  type        = list(string)
  default     = []  # Use empty list as default, not null
  description = "Additional security groups. Leave empty to skip."
}

variable "optional_tag" {
  type    = string
  default = ""  # Use empty string instead of null
  description = "Optional tag value. Leave empty to omit."
}

# Then filter out empty strings if needed:
locals {
  tags = [for tag in [var.optional_tag, "required"] : tag if tag != ""]
}