Update MFR_Adapter.java

Author: koder95

src/main/java/pl/koder95/cmf/MFR_Adapter.java

@@ -4,14 +4,15 @@
 import pl.koder95.cmf.core.MovableFeastsRow;
 
 import java.time.LocalDate;
+import java.util.Objects;
 import java.util.stream.Stream;
 
 public class MFR_Adapter extends MFR_Abstract {
 
     private final MovableFeastsRow mfr;
 
     public MFR_Adapter(MovableFeastsRow mfr) {
-        this.mfr = mfr;
+        this.mfr = Objects.requireNonNull(mfr);
     }
 
     @Override