(undefined method `year' for nil:NilClass) when saving date - ruby-on-rails

Question

I have a ruby on rails app. I added date_field to the view for add/Edit dates. the problem is that I have problem for saving and updating new dates but it brings the date from variable to the form(Edit) and after editting a date and creatine a new date. it doesn't come back to the home page or have problem with saving. as I saw in the console params, my date variable(start) changed when the user select new date but it does not load the first page afterward and I got this error:
F, [2018-02-01T16:03:44.784113 #723] FATAL -- :
NoMethodError (undefined method `year' for nil:NilClass):
app/controllers/weeks_controller.rb:27:in `create'
here is my form page where it shows date to edit:
.form-group
= f.label :course
= #course.name
.form-group.form-inline
= f.label :start
= f.date_field :start, as: :date, value: f.object.try(:strftime,"%m/%d/%Y"), class: 'form-control'
//= f.date_select :start, {}, { :class => "form-control" }
.actions
= f.submit 'Save', :class => "btn btn-primary"
an the error says about controller. but I do not have any 'year' method in controller:
def create
#week = Week.new(week_params.merge(course_id: #course.id))
respond_to do |format|
if #week.save
format.html { redirect_to '/weeks', notice: 'was successfully created.' }
format.json { render action: 'show', status: :created, location: #week }
else
format.html { render action: 'new' }
format.json { render json: #week.errors, status: :unprocessable_entity }
end
end
end
def update
respond_to do |format|
if #week.update(week_params)
format.html { redirect_to '/weeks', notice: 'starting week was successfully updated.' }
format.json { head :no_content }
else
format.html { render action: 'edit' }
format.json { render json: #week.errors, status: :unprocessable_entity }
end
end
end
weeks_params defined as follows:
def week_params
params.require(:week).permit(:start, :course_id)
end
I have at first problem of showing date with date_select but now with date_field it shows that but I do not know where this error refers to. I would be thankful if any one would help me.
The model is:
class Week < ActiveRecord::Base
belongs_to :course
has_many :assessments
def select_name
start.strftime("Week of %b %d")
end
end