First of all 2 mistakes
1. You don't need to check for NULL with LIKE
2. You must use parameterized queries.
Dim cmd As MySqlCommand = New MySqlCommand("SELECT * from inventory where Iterm like @Iterm%")
cmd.Parameters.AddWithValue("@Iterm", TextBox1.Text)
cmd.Connection = con
Dim adapater As MySqlDataAdapter = New SqlDataAdapter(cmd)