ComboBox Datasource giving exception [duplicate]

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• What is a NullReferenceException and how do I fix it? 18 answers

I have this simple piece of code for populating a ComboBox :

 DataSet dt = new DataSet();

 dt = db.getCourses(depID, academicYearValue, semID);
 if (dt.Tables[0].Rows.Count > 0)
 {
     dropdownCourses.DataSource = dt.Tables[0];
     dropdownCourses.DisplayMember = "Course";
     dropdownCourses.ValueMember = "ID";
 }

I have 4 ComboBoxes on the Form all work with the code above and get populated, only for this ComboBox when populating DataSouce i get exception :

Object reference not set to an instance of an object.

And i cannot figure out anything , any suggestions what to look for?