空引用異常

我得到一個系統空引用異常錯誤，顯示如下。 alt text 空引用異常

我對4個「dropdownlists」進行驗證。我可能做錯了什麼？

代碼如下。頁面的

protected void addExhibitButton_Click(object sender, EventArgs e) 
    { 

     if (Page.IsValid) 
     { 
      DateTime exhibitDate = DateTime.Now; 
      int caseid = Convert.ToInt32(DropDownListcaseid.SelectedItem.Text); 
      string exhibittype = exhibitTypeDropDownList.Text.ToString(); 
      string storedloc = storedLocationDropDownList.Text.ToString(); 
      string offid = DropDownList1.SelectedItem.Text.ToString(); 
      string status = "Pending"; 
      Stream imgStream = exhibitImageFileUpload.PostedFile.InputStream; 
      int imgLen = exhibitImageFileUpload.PostedFile.ContentLength;     
      byte[] imgBinaryData = new byte[imgLen]; 
      int n = imgStream.Read(imgBinaryData,0,imgLen); 
      try 
      { 
       SqlConnection connections = new SqlConnection(strConn); 
       SqlCommand command = new SqlCommand("INSERT INTO Exhibits (CaseID, ExhibitType, ExhibitImage, DateReceived, StoredLocation, InvestigationStatus, OfficerID, SuspectID, InvestigatorID, ManagerID, AdminID) VALUES (@CaseID, @ExhibitType, @ExhibitImage, @DateReceived, @StoredLocation, @InvestigationStatus, @OfficerID, @SuspectID, @InvestigatorID, @ManagerID, @AdminID)", connections); 


       SqlParameter param0 = new SqlParameter("@CaseID", SqlDbType.Int); 
       param0.Value = caseid; 
       command.Parameters.Add(param0); 

       SqlParameter param1 = new SqlParameter("@ExhibitType", SqlDbType.NText); 
       param1.Value = exhibittype; 
       command.Parameters.Add(param1); 

       SqlParameter param2 = new SqlParameter("@ExhibitImage", SqlDbType.Image); 
       param2.Value = imgBinaryData; 
       command.Parameters.Add(param2); 

       SqlParameter param3 = new SqlParameter("@DateReceived", SqlDbType.SmallDateTime); 
       param3.Value = exhibitDate; 
       command.Parameters.Add(param3); 

       SqlParameter param4 = new SqlParameter("@StoredLocation", SqlDbType.NText); 
       param4.Value = storedloc; 
       command.Parameters.Add(param4); 

       SqlParameter param5 = new SqlParameter("@InvestigationStatus", SqlDbType.VarChar, 50); 
       param5.Value = status; 
       command.Parameters.Add(param5); 

       SqlParameter param6 = new SqlParameter("@OfficerID", SqlDbType.NChar, 10); 
       param6.Value = offid; 
       command.Parameters.Add(param6); 

       SqlParameter param7 = new SqlParameter("@SuspectID", SqlDbType.NChar, 10); 
       param7.Value = DBNull.Value; 
       command.Parameters.Add(param7); 

       SqlParameter param8 = new SqlParameter("@InvestigatorID", SqlDbType.NChar, 10); 
       param8.Value = DBNull.Value; 
       command.Parameters.Add(param8); 

       SqlParameter param9 = new SqlParameter("@ManagerID", SqlDbType.NChar, 10); 
       param9.Value = DBNull.Value; 
       command.Parameters.Add(param9); 

       SqlParameter param10 = new SqlParameter("@AdminID", SqlDbType.NChar, 10); 
       param10.Value = adminID; 
       command.Parameters.Add(param10); 

       connections.Open(); 
       int numRowsAffected = command.ExecuteNonQuery(); 
       connections.Close(); 

       if (numRowsAffected != 0) 
       { 

        DropDownListcaseid.ClearSelection(); 
        exhibitTypeDropDownList.Text = null; 
        storedLocationDropDownList.Text = null; 
        DropDownList1.ClearSelection(); 
        messageLabel.Text = "Rows Inserted successfully"; 
        messageLabel.Visible = true; 
       } 
       else 
       { 
        messageLabel.Text = "An error occured while inserting columns."; 
        messageLabel.Visible = true; 

       } 
      } 
      catch (Exception ex) 
      { 
       string script = "<script>alert('" + ex.Message + "');</script>"; 
      } 

     } 

    }

圖片中運行以下 alt text