Null error in my Java Script (Error: 'null' is null or not an object)

I am using Java script to get the focus when a cursor is placed in a text box. When focus is set in the textbox, with Java Script, I set the text of a label control to empty or a space. But I am getting a null exception at line 4 (And or 6) in my Java Script. Can someone help me figure out why? This only happens if I use it for an asp.net multi line textbox?
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Line: 4: Object required (Error: 'null' is null or not an object)
 
Points to this line 4, I tried adding line 6 and removing line 4, but I get the same error.
1  <script type="text/javascript">
2     function clearNewInfoLabel() {
3       var lbl = document.getElementById('ctl00_Con ...