[ONOS-3717] Add checkNotNull for the parameters of the constructors of OchPort, OduCltPort and OmsPort.

Change-Id: I48a5735b3b421d7bbcd91e318ae7bb090d914cbc
diff --git a/core/api/src/main/java/org/onosproject/net/OchPort.java b/core/api/src/main/java/org/onosproject/net/OchPort.java
index 94d4d32..8f1630b 100644
--- a/core/api/src/main/java/org/onosproject/net/OchPort.java
+++ b/core/api/src/main/java/org/onosproject/net/OchPort.java
@@ -45,7 +45,7 @@
     public OchPort(Element element, PortNumber number, boolean isEnabled, OduSignalType signalType,
                    boolean isTunable, OchSignal lambda, Annotations... annotations) {
         super(element, number, isEnabled, Type.OCH, 0, annotations);
-        this.signalType = signalType;
+        this.signalType = checkNotNull(signalType);
         this.isTunable = isTunable;
         this.lambda = checkNotNull(lambda);
     }
diff --git a/core/api/src/main/java/org/onosproject/net/OduCltPort.java b/core/api/src/main/java/org/onosproject/net/OduCltPort.java
index f51393a..08bb943 100644
--- a/core/api/src/main/java/org/onosproject/net/OduCltPort.java
+++ b/core/api/src/main/java/org/onosproject/net/OduCltPort.java
@@ -18,6 +18,7 @@
 import java.util.Objects;
 
 import static com.google.common.base.MoreObjects.toStringHelper;
+import static com.google.common.base.Preconditions.checkNotNull;
 
 /**
  * Implementation of ODU client port (Optical channel Data Unit).
@@ -49,7 +50,7 @@
     public OduCltPort(Element element, PortNumber number, boolean isEnabled,
                       SignalType signalType, Annotations... annotations) {
         super(element, number, isEnabled, Type.ODUCLT, 0, annotations);
-        this.signalType = signalType;
+        this.signalType = checkNotNull(signalType);
     }
 
     /**
diff --git a/core/api/src/main/java/org/onosproject/net/OmsPort.java b/core/api/src/main/java/org/onosproject/net/OmsPort.java
index d37fe75..f2065fd 100644
--- a/core/api/src/main/java/org/onosproject/net/OmsPort.java
+++ b/core/api/src/main/java/org/onosproject/net/OmsPort.java
@@ -20,6 +20,7 @@
 import java.util.Objects;
 
 import static com.google.common.base.MoreObjects.toStringHelper;
+import static com.google.common.base.Preconditions.checkNotNull;
 
 /**
  * Implementation of OMS port (Optical Multiplexing Section).
@@ -49,9 +50,9 @@
     public OmsPort(Element element, PortNumber number, boolean isEnabled,
                    Frequency minFrequency, Frequency maxFrequency, Frequency grid, Annotations... annotations) {
         super(element, number, isEnabled, Type.OMS, 0, annotations);
-        this.minFrequency = minFrequency;
-        this.maxFrequency = maxFrequency;
-        this.grid = grid;
+        this.minFrequency = checkNotNull(minFrequency);
+        this.maxFrequency = checkNotNull(maxFrequency);
+        this.grid = checkNotNull(grid);
     }
 
     /**