Solution for Error "Conversion from string "" to type 'Integer' is not valid." When Closing a form

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
    TextBox1.Text = ComboBox1.SelectedValue
    Me.Salary_tableTableAdapter.FillBy(Me.Emp_mngmtDataSet.Salary_table, empno:=TextBox1.Text)
End Sub

Public Class Form3

    Private Sub Salary_tableBindingNavigatorSaveItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Salary_tableBindingNavigatorSaveItem.Click
        Me.Validate()
        Me.Salary_tableBindingSource.EndEdit()
        Me.TableAdapterManager.UpdateAll(Me.Emp_mngmtDataSet)

    End Sub

    Private Sub Form3_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'TODO: This line of code loads data into the 'Emp_mngmtDataSet.Emp_table' table. You can move, or remove it, as needed.
        Me.Emp_tableTableAdapter.Fill(Me.Emp_mngmtDataSet.Emp_table)
        'TODO: This line of code loads data into the 'Emp_mngmtDataSet.Salary_table' table. You can move, or remove it, as needed.
        If TextBox1.Text <> "" Then
            Me.Salary_tableTableAdapter.FillBy(Me.Emp_mngmtDataSet.Salary_table, empno:=TextBox1.Text)
        ElseIf TextBox1.Text = "" Then
            TextBox1.Text = ComboBox1.SelectedValue
        End If
        Me.Salary_tableTableAdapter.FillBy(Me.Emp_mngmtDataSet.Salary_table, empno:=TextBox1.Text)

    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        End
    End Sub

    Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label1.Click

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Form6.Show()
    End Sub

    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        TextBox1.Text = ComboBox1.SelectedValue
         Me.Salary_tableTableAdapter.FillBy(Me.Emp_mngmtDataSet.Salary_table, empno:=TextBox1.Text)
    End Sub
End Class