"Object reference not set to an instance of an object." - working with SharPoint 2010 fields

You can use this code to check values:

                        foreach (SPListItem row in list.Items)
                        {
                            //Checks if "Number" field contain any data
                             if (row["Number"] != null)
                                Console.WriteLine("Value of field is: {0}", row[myLookupColumnName].ToString());
                        }


or another way how to treat empty field is to use try and catch block like this:

                        foreach (SPListItem row in list.Items)
                        {
                            //Tries to catch exception
                            try
                            {
                                Console.WriteLine("Value of field is: {0}", row[myLookupColumnName].ToString());
                            }
                            catch (NullReferenceException ex)
                            {
                                Console.WriteLine("The field was empty. Error: " + ex.Message);
                            }
                        }

SharePoint List Defination Validation on custom column in Schema.xml file

There is any option Field for validation the column:-

 <Validation  Message="You must specify a value for correct answer field.">

  </Validation>

Below is the schema.xml file include custom validation in column

xml version="1.0" encoding="utf-8"?>

<List xmlns:ows="Microsoft SharePoint" Title="Question List" FolderCreation="FALSE" Direction="$Resources:Direction;" Url="Lists/QuestionList" BaseType="0" xmlns="http://schemas.microsoft.com/sharepoint/">

  <MetaData>

    <ContentTypes>

    </ContentTypes>

    <Fields>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F5673F9}" Name="Title" DisplayName="Question" Type="Text" MaxLength="256" Required="TRUE"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F5773F9}" Name="Answer1" DisplayName="Answer 1" Type="Text" MaxLength="256" Required="TRUE"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F5873F9}" Name="Answer2" DisplayName="Answer 2" Type="Text" MaxLength="256" Required="TRUE"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F5973F9}" Name="Answer3" DisplayName="Answer 3" Type="Text" MaxLength="256"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6073F9}" Name="Answer4" DisplayName="Answer 4" Type="Text" MaxLength="256"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6173F9}" Name="Answer5" DisplayName="Answer 5" Type="Text" MaxLength="256"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6273F9}" Name="Answer6" DisplayName="Answer 6" Type="Text" MaxLength="256"></Field>

      <Field Type="Choice"  DisplayName="Correct Answer" Required="TRUE" EnforceUniqueValues="FALSE" Indexed="FALSE" Format="Dropdown" FillInChoice="FALSE"

       ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6373F9}"  Name="Correct Answer"  Version="1">

        <CHOICES>

          <CHOICE>1</CHOICE>

          <CHOICE>2</CHOICE>

          <CHOICE>3</CHOICE>

          <CHOICE>4</CHOICE>

          <CHOICE>5</CHOICE>

          <CHOICE>6</CHOICE>

        </CHOICES>

        <Validation  Message="You must specify a value for correct answer field.">

          =IF([Correct Answer]="3",IF(ISBLANK([Answer3]),False,True),IF([Correct Answer]="4",IF(ISBLANK([Answer4]),False,True),IF([Correct Answer]="5",IF(ISBLANK([Answer5]),False,True),IF([Correct Answer]="6",IF(ISBLANK([Answer6]),False,True),True)))

        </Validation>

      </Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6473F9}" Name="CorrectAnswer" DisplayName="Correct Answer"  ShowInDisplayForm="FALSE" ShowInEditForm="FALSE" ShowInNewForm="FALSE" Type="Text" MaxLength="256"  Required="TRUE"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E9169F6573F9}" Name="DisplayOrder" DisplayName="DisplayOrder" Type="Number" Decimals="0"  Required="TRUE"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6673F9}" Name="Active" DisplayName="Active" Type="Boolean"  Required="TRUE"></Field>

      <Field ID="{8BD30E52-9D36-4CDC-BEBB-E8169F6773F9}" Name="AnswerDescription" DisplayName="Answer Description" Type="Note"></Field>

    </Fields>

    <Views>

      <View BaseViewID="0" Type="HTML" MobileView="TRUE" TabularView="FALSE">

        <Toolbar Type="Standard" />

        <XslLink Default="TRUE">main.xsl</XslLink>

        <RowLimit Paged="TRUE">30</RowLimit>

        <ViewFields>

          <FieldRef Name="LinkTitleNoMenu"></FieldRef>

        </ViewFields>

        <Query>

          <OrderBy>

            <FieldRef Name="Modified" Ascending="FALSE"></FieldRef>

          </OrderBy>

        </Query>

        <ParameterBindings>

          <ParameterBinding Name="AddNewAnnouncement" Location="Resource(wss,addnewitem)" />

          <ParameterBinding Name="NoAnnouncements" Location="Resource(wss,noXinviewofY_LIST)" />

          <ParameterBinding Name="NoAnnouncementsHowTo" Location="Resource(wss,noXinviewofY_ONET_HOME)" />

        </ParameterBindings>

      </View>

      <View BaseViewID="1" Type="HTML" WebPartZoneID="Main" DisplayName="$Resources:core,objectiv_schema_mwsidcamlidC24;" DefaultView="TRUE" MobileView="TRUE" MobileDefaultView="TRUE" SetupPath="pages\viewpage.aspx" ImageUrl="/_layouts/images/generic.png" Url="AllItems.aspx">

        <Toolbar Type="Standard" />

        <XslLink Default="TRUE">main.xsl</XslLink>

        <RowLimit Paged="TRUE">30</RowLimit>

        <ViewFields>

          <FieldRef Name="ID"></FieldRef>

          <FieldRef Name="Title"></FieldRef>

          <FieldRef Name="Answer1"></FieldRef>

          <FieldRef Name="Answer2"></FieldRef>

          <FieldRef Name="Answer3"></FieldRef>

          <FieldRef Name="Answer4"></FieldRef>

          <FieldRef Name="Answer5"></FieldRef>

          <FieldRef Name="Answer6"></FieldRef>

          <FieldRef Name="CorrectAnswer"></FieldRef>

          <FieldRef Name="Active"></FieldRef>

          <FieldRef Name="AnswerDescription"></FieldRef>

          <FieldRef Name="DisplayOrder"></FieldRef>

          <FieldRef Name="Author"></FieldRef>

          <FieldRef Name="Editor"></FieldRef>

        </ViewFields>

        <Query>

          <OrderBy>

            <FieldRef Name="ID"></FieldRef>

          </OrderBy>

        </Query>

        <ParameterBindings>

          <ParameterBinding Name="NoAnnouncements" Location="Resource(wss,noXinviewofY_LIST)" />

          <ParameterBinding Name="NoAnnouncementsHowTo" Location="Resource(wss,noXinviewofY_DEFAULT)" />

        </ParameterBindings>

      </View>

    </Views>

    <Forms>

      <Form Type="DisplayForm" Url="DispForm.aspx" SetupPath="pages\form.aspx" WebPartZoneID="Main" />

      <Form Type="EditForm" Url="EditForm.aspx" SetupPath="pages\form.aspx" WebPartZoneID="Main" />

      <Form Type="NewForm" Url="NewForm.aspx" SetupPath="pages\form.aspx" WebPartZoneID="Main" />

    </Forms>

  </MetaData>

</List>