Fix NPE caused by fix for Grid resizing with split panel (#18370)

Change-Id: Ic8b5898facf559f253357e6cde6ee2b10be63a79

client/src/com/vaadin/client/ui/VSplitPanelHorizontal.java

@@ -28,11 +28,11 @@ public class VSplitPanelHorizontal extends VAbstractSplitPanel {
 
     @Override
     protected void startResize() {
-        if (isWidgetFullWidth(getFirstWidget())) {
+        if (getFirstWidget() != null && isWidgetFullWidth(getFirstWidget())) {
             getFirstContainer().getStyle().setOverflow(Overflow.HIDDEN);
         }
 
-        if (isWidgetFullWidth(getSecondWidget())) {
+        if (getSecondWidget() != null && isWidgetFullWidth(getSecondWidget())) {
             getSecondContainer().getStyle().setOverflow(Overflow.HIDDEN);
         }
     }

client/src/com/vaadin/client/ui/VSplitPanelVertical.java

@@ -28,11 +28,11 @@ public class VSplitPanelVertical extends VAbstractSplitPanel {
 
     @Override
     protected void startResize() {
-        if (isWidgetFullHeight(getFirstWidget())) {
+        if (getFirstWidget() != null && isWidgetFullHeight(getFirstWidget())) {
             getFirstContainer().getStyle().setOverflow(Overflow.HIDDEN);
         }
 
-        if (isWidgetFullHeight(getSecondWidget())) {
+        if (getSecondWidget() != null && isWidgetFullHeight(getSecondWidget())) {
             getSecondContainer().getStyle().setOverflow(Overflow.HIDDEN);
         }
     }