We have a client website where dropdowns stop working.

Google Console shows the following:

Uncaught TypeError: Cannot read properties of null (reading 'style')
at OpenListBox (Location_Edit.aspx?lid=11502:47)
at HTMLSpanElement.onclick (Location_Edit.aspx?lid=11502:623)

Code for this section is as follows: ** is line 623 where error is noted.

<div>
    <table id="canvas_multiSelectDropDown1_Table2" 
           style="TABLE-LAYOUT: fixed; HEIGHT: 24px" 
           borderColorDark="#cccccc" borderColorLight="#7eb3e3" 
           cellspacing="0" cellpadding="0" width="100%" 
           border="1">
        <tr id="canvas_multiSelectDropDown1_rowDD" ** style="HEIGHT: 15px">
            <TD nowrap="nowrap">
                    <span id="canvas_multiSelectDropDown1_DDLabel" onclick="OpenListBox(&#39;ctl00_canvas_multiSelectDropDown1_&#39;);" style="display:inline-block;border-color:Transparent;border-style:None;font-family:Arial;font-size:Smaller;height:15px;width:250px;CURSOR: default">VSP  - Xerox Phaser 6600 (Black)</span></TD>**
            <TD id="canvas_multiSelectDropDown1_colDDImage" style="PADDING-RIGHT: 0px; PADDING-LEFT: 0px; PADDING-BOTTOM: 0px; PADDING-TOP: 0px; BACKGROUND-COLOR: #7eb3e3" width="20" background="images/DDImage.bmp" onclick="OpenListBox(&#39;ctl00_canvas_multiSelectDropDown1_&#39;);"></TD>
        </tr>
    </table>

Looking further I ....is it possible error is in the set of code: OpenListBox

    function OpenListBox(ctlId) {
        var lstBox = document.getElementById(ctlId + "DDList");
        if (lstBox.style.visibility == "visible")
        { CloseListBox(ctlId); }
        else {
            lstBox.style.visibility = "visible";
            lstBox.style.height = "120px";
        }
    }

_________________-

Issue solved: This was a recently migrated website- after migrating I noticed: Current verison iss Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.7.3905.0

The solution was to roll back to Microsoft .NET Framework version 2.0