I am having this same issue and have been pulling my hair trying to figure this out.
I am actually running this on a CentOS box.
I created a spreadsheet with text in one cell, loaded the file into a "reader" and "saved it" with a "writer" and it worked fine.
Modified the spreadsheet (in Excel) and added an image, saved the spreadsheet to the same name. Ran through the same test, and I got the same type error that you did and it actually "deleted" my spreadsheet. (Luckily, I maintained a copy
for further testing).
Modified the spreadsheet (in Excel) again and removed the image, save the spreadsheet to the same name. Ran through the same test and it worked fine.
In playing around I noticed that with or without the image the value of $writer->getDrawingHashTable()->count() is 0. Not sure if that means anything.
I am not sure where to go from here.