MVC Url.Action causes NullReferenceException in post - asp.net-mvc

Question

I'm trying to have a button on my form page that takes me back to the previous view that requires a parameter, I'm passing a ViewModel into the view with the assigned value I'm using. If the button is un-commented the button works fine but the forms post sends a NullReferenceException, and if the button is commented the form works exactly as I want.
The button that breaks the form
<button type="button" onclick="location.href='#Url.Action("Assignments","Session", new { Model.CourseName })'">Go Back</button>
The Controller Code
public IActionResult CreateAssignment(string courseName)
{
CreateAssignmentModel assignmentModel = new CreateAssignmentModel();
assignmentModel.CourseName = courseName;
return View(assignmentModel);
}
[HttpPost]
public IActionResult CreateAssignment(CreateAssignmentModel assignment)
{
if (ModelState.IsValid)
{
ModelState.Clear();
return View(assignment.CourseName);
}
else
{
return View(assignment.CourseName);
}
}
public IActionResult Assignments(string courseName)
{
var assignments = storedProcedure.getAssignments(User.Identity.Name, courseName);
var AssignmentsView = new AssignmentsViewModel{CourseName = courseName};
foreach (var Assignment in assignments.ToList())
{
AssignmentsView.Assignments.Add(Assignment);
}
return View(AssignmentsView);
}
The Model Code
public class CreateAssignmentModel
{
public string UserName { get; set; }
public string CourseName { get; set; }
[Required]
public string AssignmentName { get; set; }
[Required]
public string AssignmentDescription { get; set; }
[Required]
public int TotalPoints { get; set; }
[Required]
public DateTime DueDate { get; set; }
}
The Form with Button
<button type="button" onclick="location.href='#Url.Action("Assignments","Session", new { Model.CourseName })'">Go Back</button>
<div class="row">
<div class="col-md-4">
<form asp-route-returnUrl="#ViewData["ReturnUrl"]" method="post">
<h4>Create an Assignment</h4>
<hr />
<div class="form-group">
<label asp-for="AssignmentName" class="control-label">Assignment Name</label>
<input asp-for="AssignmentName" class="form-control" placeholder="Assignment Name" />
<span asp-validation-for="AssignmentName" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="AssignmentDescription" class=" control-label">Assignment Description</label>
<input asp-for="AssignmentDescription" class="form-control" placeholder="Assignment Description" />
<span asp-validation-for="AssignmentDescription" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="TotalPoints" class=" control-label">Total Points</label>
<input asp-for="TotalPoints" class="form-control" placeholder="Points" />
<span asp-validation-for="TotalPoints" class="text-danger"></span>
</div>
<div class="form-group">
<label asp-for="DueDate" class=" control-label">Due Date</label>
<input asp-for="DueDate" class="form-control" placeholder="Due Date" />
<span asp-validation-for="DueDate" class="text-danger"></span>
</div>
<br />
<button type="submit" class="btn btn-primary">Create</button>
</form>
</div>
</div>
Sorry for the lack of brevity, I've looked at NullReferenceException solutions for my problem but none have worked.