The method getWebRootDirLocation() does not exist on Sunnysideup\UpgradeToSilverstripe4\ModuleUpgrader. Since you implemented __call, consider adding a @method annotation.
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
Are you sure $this->mu()->getWebRootDirLocation() of type Sunnysideup\UpgradeToSil...duleUpgrader|mixed|null can be used in concatenation?
(
Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation