Fix exception

app/controllers/circuits_controller.rb

@@ -1,10 +1,12 @@
 class CircuitsController < ApplicationController
   def index
-    @circuits = Circuit.all.sort_by(&:average_grade)
+    @circuits = Circuit.all.select { |c| c.problems.any? }.sort_by(&:average_grade)
   end
 
   def show
     @circuit = Circuit.find(params[:id])
+    redirect_to circuits_path unless @circuit.problems.any?
+
     @area = @circuit.main_area
   end
 end

app/models/circuit.rb

@@ -17,7 +17,7 @@ def name
   end
 
   def main_area
-    problems.order(:circuit_number).first.area
+    problems.order(:circuit_number).first&.area
   end
 
   def bounds
@@ -53,6 +53,7 @@ def beginner_friendly?
 
   private
   def average(array)
+    return 0 if array.empty?
     array.reduce(:+) / array.size.to_f
   end
 end