Rails and ajax file upload - cannot read property 'innerHTML' of null error

This is old but I found an actual solution to this. Change your require in application.js to //=require jquery-fileupload/basic instead of //= require jquery-fileupload.

The reason you're getting this error is because you are missing a script tag id="template-upload" or id="template-download" or both.

Example from the Demo at: http://blueimp.github.com/jQuery-File-Upload/

<script id="template-upload" type="text/x-tmpl">{% for (var i=0, file; file=o.files[i]; i++) { %}    <tr class="template-upload fade">        <td class="preview"><span class="fade"></span></td>        <td class="name"><span>{%=file.name%}</span></td>        <td class="size"><span>{%=o.formatFileSize(file.size)%}</span></td>        {% if (file.error) { %}            <td class="error" colspan="2"><span class="label label-important">Error</span> {%=file.error%}</td>        {% } else if (o.files.valid && !i) { %}            <td>                <div class="progress progress-success progress-striped active" role="progressbar" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0"><div class="bar" style="width:0%;"></div></div>            </td>            <td class="start">{% if (!o.options.autoUpload) { %}                <button class="btn btn-primary">                    <i class="icon-upload icon-white"></i>                    <span>Start</span>                </button>            {% } %}</td>        {% } else { %}            <td colspan="2"></td>        {% } %}        <td class="cancel">{% if (!i) { %}            <button class="btn btn-warning">                <i class="icon-ban-circle icon-white"></i>                <span>Cancel</span>            </button>        {% } %}</td>    </tr>{% } %}</script><!-- The template to display files available for download --><script id="template-download" type="text/x-tmpl">{% for (var i=0, file; file=o.files[i]; i++) { %}    <tr class="template-download fade">        {% if (file.error) { %}            <td></td>            <td class="name"><span>{%=file.name%}</span></td>            <td class="size"><span>{%=o.formatFileSize(file.size)%}</span></td>            <td class="error" colspan="2"><span class="label label-important">Error</span> {%=file.error%}</td>        {% } else { %}            <td class="preview">{% if (file.thumbnail_url) { %}                <a href="{%=file.url%}" title="{%=file.name%}" data-gallery="gallery" download="{%=file.name%}"><img src="{%=file.thumbnail_url%}"></a>            {% } %}</td>            <td class="name">                <a href="{%=file.url%}" title="{%=file.name%}" data-gallery="{%=file.thumbnail_url&&'gallery'%}" download="{%=file.name%}">{%=file.name%}</a>            </td>            <td class="size"><span>{%=o.formatFileSize(file.size)%}</span></td>            <td colspan="2"></td>        {% } %}        <td class="delete">            <button class="btn btn-danger" data-type="{%=file.delete_type%}" data-url="{%=file.delete_url%}"{% if (file.delete_with_credentials) { %} data-xhr-fields='{"withCredentials":true}'{% } %}>                <i class="icon-trash icon-white"></i>                <span>Delete</span>            </button>            <input type="checkbox" name="delete" value="1">        </td>    </tr>{% } %}</script>

The UI version depends on both these templates being present.

I was having the same problem and decided to checkout what was missing.

Hope that helps.

I ran into this same problem as well. However, loading the basic version of the file-upload plugin was not an option for me since I use previewing for one upload, but not for the other. You can disable the "preview" functionality and the attempted processing of template-upload and template-download on a case-by-case basis by setting the uploadTemplateId and downloadTemplateId options for the file-upload plugin to null.