The property DatabaseTest1 does not exist on WebStream\Test\Controller\DatabaseTest1Controller. Since you implemented __get, consider adding a @property annotation.
If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
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/** @scrutinizer ignore-call */
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$data = $this->DatabaseTest1->getData();
This check looks for calls to methods that do not seem to exist on a given type.
It looks for the method on the type itself as well as in inherited classes or
implemented interfaces.
This is most likely a typographical error or the method has been renamed.