Typically any errors that pop-up when opening a template file (or entering the design step of the campaign process) is due to "bad", "junk", or "messy" HTML code. The code may have been pasted to create the design or it may have been uploaded as a HTML file.
When the HTML editor runs code a filter, the bad code throws an error. The code filter is executed when opening, closing or switching between design and source views.
The key indicator that the error is related to bad code is the inclusion of "Error while executing filter"... Following are examples of errors, but not limited to only these two. In any case, these types of errors are seen when using the HTML editor only.
"Error while executing filter RadCellFilter-TypeError:heightArray is null"
This usually means that a HTML tag height attribute was undeclared or declared incorrectly for that cell. For example, <td tag that has the height attribute set to "auto". Removing this attribute corrects the issue.
"Error while executing filter Hyperlink filter - TypeError: element style is undefined"
This usually means that there is an issue with the hyperlink code.
If you are unable to identify the cause of the error after reviewing your HTML, please reach out to Support for assistance.