MprPushConfiguration.getPushMode gives NPE after switching from Vaadin 7 to 14

Question

I am migrating a Vaadin v7 application using MPR to Vaadin 14. We are using push mode "manual". I migrated the push mode by using Flow imports. However, I am getting a NullPointerException at runtime:

java.lang.NullPointerException
    at com.vaadin.mpr.core.MprPushConfiguration.getPushMode(MprPushConfiguration.java:31)
    at com.vaadin.server.VaadinSession.unlock(VaadinSession.java:1000)
    at com.vaadin.server.VaadinService.requestEnd(VaadinService.java:1382)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1449)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:380)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:779)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
    at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:239)
    at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:215)