You can download and examine the BarcodeScanner Sample app we provide and see if the issue can be reproduced there. Also, please note that the BarcodeScanner plugin is not supported within the AppBuilder device simulator as described in the Simulator Limitations article. This means that you will not be able to test the plugin functionality within the simulator. Also, while testing it on a device, you need to make sure that you do so after the deviceready() event was fired.
If the above information doesn't help, we would appreciate it if you can try to reproduce the issue in the sample project and share the steps we can follow to do that on our side as well. This way we will be able to test the plugin behavior locally and further investigate the case.
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