NullPointerException BillingManager.java from In-app Billing

Question

I added a new billing library to my project.

compile 'com.android.billingclient:billing:1.0'

I added a BillingManager.java class to my project from here https://github.com/googlesamples/android-play-billing/blob/master/TrivialDrive_v2/shared-module/src/main/java/com/example/billingmodule/billing/BillingManager.java

I get many exceptions, details are from Crashlytics:

Fatal Exception: java.lang.RuntimeException
Unable to resume activity {com.domain.app/com.domain.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference 

Caused by java.lang.NullPointerException
Attempt to invoke interface method 'int java.util.List.size()' on a null object reference

com.domain.app.BillingManager$4.run
com.domain.app.BillingManager.executeServiceRequest (Unknown Source)
com.domain.app.BillingManager.executeServiceRequest (Unknown Source)
com.domain.app.MainActivity.onResume (Unknown Source)

TXT mode:

Fatal Exception: java.lang.RuntimeException: Unable to resume activity {com.domain.app/com.domain.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
       at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3647)
       at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3687)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1629)
       at android.os.Handler.dispatchMessage(Handler.java:105)
       at android.os.Looper.loop(Looper.java:156)
       at android.app.ActivityThread.main(ActivityThread.java:6623)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:942)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:832)

OnResume() of MainActivity contains a following code:

if (mBillingManager != null && mBillingManager.getBillingClientResponseCode() == BillingClient.BillingResponse.OK) {
                mBillingManager.queryPurchases();
            }

How can I fix it? Around 1% of DAU have a such problem.