[MINOR][ML] When trainingSummary is None, it should throw RuntimeException.

## What changes were proposed in this pull request?
When trainingSummary is None, it should throw ```RuntimeException```.
cc mengxr
## How was this patch tested?
Existing tests.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #11784 from yanboliang/fix-summary.

4 files changed, 8 insertions, 20 deletions

diff --git a/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
index 77e59d9188..861b1d4b66 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
@@ -509,12 +509,8 @@ class LogisticRegressionModel private[spark] (
    * thrown if `trainingSummary == None`.
    */
   @Since("1.5.0")
-  def summary: LogisticRegressionTrainingSummary = trainingSummary match {
-    case Some(summ) => summ
-    case None =>
-      throw new SparkException(
-        "No training summary available for this LogisticRegressionModel",
-        new NullPointerException())
+  def summary: LogisticRegressionTrainingSummary = trainingSummary.getOrElse {
+    throw new SparkException("No training summary available for this LogisticRegressionModel")
   }
 
   /**
diff --git a/mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala b/mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala
index ab00127899..38428826a8 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala
@@ -148,12 +148,9 @@ class KMeansModel private[ml] (
    * thrown if `trainingSummary == None`.
    */
   @Since("2.0.0")
-  def summary: KMeansSummary = trainingSummary match {
-    case Some(summ) => summ
-    case None =>
-      throw new SparkException(
-        s"No training summary available for the ${this.getClass.getSimpleName}",
-        new NullPointerException())
+  def summary: KMeansSummary = trainingSummary.getOrElse {
+    throw new SparkException(
+      s"No training summary available for the ${this.getClass.getSimpleName}")
   }
 }
 
diff --git a/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
index 6e74cb54ad..0e71e8d8e1 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala
@@ -681,8 +681,7 @@ class GeneralizedLinearRegressionModel private[ml] (
   @Since("2.0.0")
   def summary: GeneralizedLinearRegressionSummary = trainingSummary.getOrElse {
     throw new SparkException(
-      "No training summary available for this GeneralizedLinearRegressionModel",
-      new RuntimeException())
+      "No training summary available for this GeneralizedLinearRegressionModel")
   }
 
   private[regression] def setSummary(summary: GeneralizedLinearRegressionSummary): this.type = {
diff --git a/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala b/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala
index c8f3f70a9b..b81c588e44 100644
--- a/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala
+++ b/mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala
@@ -398,12 +398,8 @@ class LinearRegressionModel private[ml] (
    * thrown if `trainingSummary == None`.
    */
   @Since("1.5.0")
-  def summary: LinearRegressionTrainingSummary = trainingSummary match {
-    case Some(summ) => summ
-    case None =>
-      throw new SparkException(
-        "No training summary available for this LinearRegressionModel",
-        new NullPointerException())
+  def summary: LinearRegressionTrainingSummary = trainingSummary.getOrElse {
+    throw new SparkException("No training summary available for this LinearRegressionModel")
   }
 
   private[regression] def setSummary(summary: LinearRegressionTrainingSummary): this.type = {