Protected method not accessible from subclass

Question

I know this has been asked before but my problem is strange and I couldn't find an answer.

I have a parent class where a member is declared protected

class BaseContoller<G: BaseContract.BasePresenter, F: BaseModel> {
    protected var presenter: G? = null
    // ...
}

In each of BaseController's descendants, I refer to presenter with no problem as it is accessible in the same package and subclasses.

One BaseController's descendant has to be extended by other classes too but I have to do one check before of performing certain actions in where presenter is referenced again

class BackupPrinterController<G: BackupPrinterContract.Presenter, F: BaseModel>: BackupController<G, F>
    private fun startScan() {
        if(this is PrinterAssociationController) {
            PrinterConnection.onConnection = { presenter?.onDeviceConnected(it) }
            // ...
        }
    }
}

The strange behaviour comes here, presenter is perfectly accessible only without the filter if(this is PrinterAssociationController), with that if statement, when the onConnection callback is invoked, an IllegalAccessError exception is thrown by presenter

Here is the declaration of PrinterAssociationController

class PrinterAssociationController: BackupPrinterController<PrinterAssociationContract.Presenter, PrinterAssociationModel>(),
PrinterAssociationContract.View

and here is the exception

java.lang.IllegalAccessError: Method 'package.common.base.BaseContract$BasePresenter package.common.base.BaseController.getPresenter()' is inaccessible to class 'package.project.features.shared.controllers.BackupPrinterController$startScan$1' (declaration of 'package.project.features.shared.controllers.BackupPrinterController$startScan$1' appears in /data/app/package.project-1/base.apk:classes2.dex)
    at package.project.features.shared.controllers.BackupPrinterController$startScan$1.invoke(BackupPrinterController.kt:53)
    at package.project.features.shared.controllers.BackupPrinterController$startScan$1.invoke(BackupPrinterController.kt:35)
    at package.project.utils.print.PrinterConnection$scan$1$4.accept(PrinterConnection.kt:84)
    at package.project.utils.print.PrinterConnection$scan$1$4.accept(PrinterConnection.kt:24)
    at io.reactivex.internal.observers.LambdaObserver.onNext(LambdaObserver.java:63)
    at io.reactivex.internal.operators.observable.ObservableFilter$FilterObserver.onNext(ObservableFilter.java:52)
    at io.reactivex.internal.operators.observable.ObservableDistinct$DistinctObserver.onNext(ObservableDistinct.java:85)
    at io.reactivex.internal.operators.mixed.CompletableAndThenObservable$AndThenObservableObserver.onNext(CompletableAndThenObservable.java:65)
    at io.reactivex.internal.operators.observable.ObservableFlatMap$MergeObserver.tryEmit(ObservableFlatMap.java:265)
    at io.reactivex.internal.operators.observable.ObservableFlatMap$InnerObserver.onNext(ObservableFlatMap.java:562)
    at io.reactivex.internal.operators.observable.ObservableDoOnEach$DoOnEachObserver.onNext(ObservableDoOnEach.java:101)
    at io.reactivex.internal.operators.observable.ObservableMap$MapObserver.onNext(ObservableMap.java:62)
    at io.reactivex.internal.operators.observable.ObservableUnsubscribeOn$UnsubscribeObserver.onNext(ObservableUnsubscribeOn.java:60)
    at io.reactivex.internal.operators.observable.ObservableCreate$CreateEmitter.onNext(ObservableCreate.java:66)
    at com.polidea.rxandroidble2.internal.serialization.FIFORunnableEntry$1.onNext(FIFORunnableEntry.java:66)
    at io.reactivex.internal.operators.observable.ObservableUnsubscribeOn$UnsubscribeObserver.onNext(ObservableUnsubscribeOn.java:60)
    at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeOnObserver.onNext(ObservableSubscribeOn.java:58)
    at io.reactivex.internal.operators.observable.ObservableCreate$CreateEmitter.onNext(ObservableCreate.java:66)
    at com.polidea.rxandroidble2.internal.operations.ScanOperationApi21$1.onScanResult(ScanOperationApi21.java:77)
    at android.bluetooth.le.BluetoothLeScanner$BleScanCallbackWrapper$1.run(BluetoothLeScanner.java:471)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at com.osama.firecrasher.FireLooper.run(FireLooper.kt:39)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6780)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1500)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1390)

I don't understand why this is happening when I put that if statement