At some point of a (re)compilation process of a .tex file, if the
embedded viewer is open, a warning appears in a pop-up window
does not look like a valid PDF document.
Either the file is corrupt or it is in the process of creation. You
may retry after compilation is finished. Opening a corrupt document
could cause a crash. Do you want to open it anyway?
So far, so good¹. But, at least on Linux, if the process is long
enough, multiple windows, all with the same warn message, appear (for
instance, 18 windows for a 60 pages document compiled with
latexmk). This is useless and very annoying.
IMHO, TXS should check if such a windows is already open and, if so,
not open new one(s).
¹I'm not sure pop-up windows are necessary here: a message displayed
in the PDF viewer area should be enough.