Uncaught TypeError: Cannot read property 'toLowerCase' of null - HTML5

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This error seems to be coming from the main Js file from this line, also after the change package the file using gzip( This worked for me, This helped me (HTML5 uncaught typeError: cannot read property 'toLowerCase' of null - Platform & Builds - Epic Developer Community Forums):

                 // After enabling Content-Encoding: gzip, make sure that the appropriate MIME type is being used for the asset, i.e. the MIME
                 // type should be that of the uncompressed asset, and not the MIME type of the compression method that was used.
                 if (xhr.getResponseHeader('Content-Type').toLowerCase().indexOf('zip') != -1) {

There “xhr.getResponseHeader(‘Content-Type’)” is returning null. Try changing the if statement to

                 if (xhr.getResponseHeader('Content-Type') && xhr.getResponseHeader('Content-Type').toLowerCase().indexOf('zip') != -1) {