http://localhost/undefined 404 (Not Found)
I found where this issue could appear from.
- Make sure that all the generated CSS (styles) that set a background-image or the like, where there's an url() actually point somewhere; if it doesn't point anywhere the error will set to the .html DOCTYPE.
- Check the src of your image tags; make sure that they are not set by JavaScript to undefined.
- Also check the src of your script files, even when it'd be much rarer in that case to happen.
So yes it's awful that it actually doesn't point where the actual error is but in fact has to be with a lookup resource that it isn't able to find; we probably don't even want it to search. Eg. when the element is invisible and we don't set any image; but we still pass undefined it'll search for undefined and it'll not give you any hint of in which part of the tree the error is.
Since your page is too complex to know where it is, or if this is even the problem you are facing; this is the most I can tell; the only reason I could find it too quick was because of react dev tools told me right away of my render properties and the fact that the component was quite small.
One other way to find what is causing this issue (when you have a decent sized solution) is to check if there is any undefined assigned to any html attributes/styles. We can check this by right clicking on body tag, Expand recursively, copy the HTML, paste it and search for undefined.
If there is undefined assigned to href/src/url etc. it might give you a clue on what is wrong.
This can be fixed by not assigning undefined to those attributes.