One of the extension which you are using is not compatible with Android X
java.lang.NoClassDefFoundError: Failed resolution of: Landroid/support/v4/view/ViewPager;
at java.lang.Class.getDeclaredFields(Native Method)
at gnu.bytecode.ClassType.addFields(ClassType.java:606)
at gnu.bytecode.ClassType.getFields(ClassType.java:506)
at gnu.bytecode.ClassType.hasOuterLink(ClassType.java:260)
at gnu.bytecode.ClassType.getDeclaredMethod(ClassType.java:880)
at gnu.bytecode.ClassType.addMethod(ClassType.java:667)
at gnu.bytecode.ClassType.addMethod(ClassType.java:698)
at gnu.bytecode.ClassType.addMethods(ClassType.java:1009)
at gnu.bytecode.ClassType.getDeclaredMethods(ClassType.java:725)
at gnu.bytecode.ClassType.getMethods(ClassType.java:809)
at gnu.kawa.reflect.ClassMethods.getMethods(ClassMethods.java:106)
at gnu.kawa.reflect.ClassMethods.apply(ClassMethods.java:229)
at gnu.kawa.reflect.Invoke.lookupMethods(Invoke.java:278)
at gnu.kawa.reflect.Invoke.applyN(Invoke.java:185)
at gnu.mapping.ProcedureN.apply2(ProcedureN.java:39)
at io.kodular.androlartech.Insta_saver.POST.$define(POST.yail:10250)
at com.google.appinventor.components.runtime.Form.onCreateFinish2(SourceFile:502)
at com.google.appinventor.components.runtime.Form.onCreateFinish(SourceFile:446)
at com.google.appinventor.components.runtime.Form.onCreate(SourceFile:385)
at android.app.Activity.performCreate(Activity.java:7893)
at android.app.Activity.performCreate(Activity.java:7880)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1307)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3286)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3460)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2047)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:224)
at android.app.ActivityThread.main(ActivityThread.java:7590)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
Caused by: java.lang.ClassNotFoundException: android.support.v4.view.ViewPager