html - Javascript TypeError with IFrame -

I'm trying to change a IFrame twice with the click of a link. Uncaught TypeError :: I can not always get this error receiving the property's 'src' of the null whenever it is clicked on that link

what is needed to do this, The source is changing. Change the source of code> IFrame on the index page, and then on a IFrame page that was loaded in to the external IFrame index page.

  & lt; Div class = "topics list-group" target = "../../ body.html" & gt; & Lt; Script & gt; Function doneLoading () {document.getElementById ('index.html # large-column') src = 'body.html'; } & Lt; / Script & gt; & Lt; A href = "topic / english / course php" onclick = 'doneLoading ();' Target = 'topic' class = "list-group-item" & gt; & Lt; Span class = "badge" & gt; 26 & lt; / Span & gt; English & lt; / A & gt; & Lt; / Div & gt; Text after a  

A cleaner approach will be used by iFrame targeting.

  & lt; Iframe id = 'content' src = 'body-default.html' & gt; & Lt; A href = 'body-en.html' target = 'content' & gt; English & lt; / A & gt;  

Body-dynamic.html or body-en.html files may be required to update themselves if necessary.