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Mostly it happens that you are working fine with your WordPress theme. Your nav-menus.php page is also working fine. But after the installation of some new plugin, you can observe that sometime nav-menus.php page fails to load.
Here, we are providing a solution to overcome this problem.
De-activate the newly installed plugin and check again.
After de-activating the newly installed plugin, the problem is not solved, then you can de-activate all the plugins in bulk, and then deactivate all the plugin one by one.