Fix TabControl NREs (#2831)

src/System.Windows.Forms/src/Resources/SR.resx

@@ -5336,6 +5336,9 @@ Stack trace where the illegal operation occurred was:
   <data name="TabBaseTabCountDescr" xml:space="preserve">
     <value>The number of tabs in the tab strip.</value>
   </data>
+  <data name="TabControlBadControl" xml:space="preserve">
+    <value>Parameter must be of type TabPage.</value>
+  </data>
   <data name="TabControlDeselectedEventDescr" xml:space="preserve">
     <value>Occurs after a tab page is deselected as the topmost tab page.</value>
   </data>

src/System.Windows.Forms/src/System/Windows/Forms/TabControl.cs

@@ -786,31 +786,17 @@ public int SelectedIndex
         SRDescription(nameof(SR.TabControlSelectedTabDescr))
         ]
         public TabPage SelectedTab
-        {
-            get
-            {
-                return SelectedTabInternal;
-            }
-            set
-            {
-                SelectedTabInternal = value;
-            }
-        }
-
-        internal TabPage SelectedTabInternal
         {
             get
             {
                 int index = SelectedIndex;
-                if (index == -1)
+                if (index == -1 || _tabPages == null)
                 {
                     return null;
                 }
-                else
-                {
-                    Debug.Assert(0 <= index && index < _tabPages.Length, "SelectedIndex returned an invalid index");
-                    return _tabPages[index];
-                }
+
+                Debug.Assert(0 <= index && index < _tabPages.Length, "SelectedIndex returned an invalid index");
+                return _tabPages[index];
             }
             set
             {
@@ -1228,7 +1214,16 @@ public Rectangle GetTabRect(int index)
 
         protected string GetToolTipText(object item)
         {
-            return ((TabPage)item).ToolTipText;
+            if (item == null)
+            {
+                throw new ArgumentNullException(nameof(item));
+            }
+            if (!(item is TabPage tabPage))
+            {
+                throw new ArgumentException(SR.TabControlBadControl, nameof(item));
+            }
+
+            return tabPage.ToolTipText;
         }
 
         private void ImageListRecreateHandle(object sender, EventArgs e)

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/TabControlTests.cs

@@ -2527,16 +2527,16 @@ public void TabControl_SelectedTab_GetWithPagesWithHandle_ReturnsExpected()
         [InlineData(0)]
         [InlineData(1)]
         [InlineData(2)]
-        public void TabControl_SelectedTab_GetWithInvalidIndexNotGotPages_ThrowsNullReferenceException(int value)
+        public void TabControl_SelectedTab_GetWithInvalidIndexNotGotPages_ReturnsNull(int value)
         {
             using var control = new TabControl
             {
                 SelectedIndex = value
             };
-            Assert.Throws<NullReferenceException>(() => control.SelectedTab);
+            Assert.Null(control.SelectedTab);
 
             Assert.Empty(control.TabPages);
-            Assert.Throws<NullReferenceException>(() => control.SelectedTab);
+            Assert.Null(control.SelectedTab);
         }
 
         public static IEnumerable<object[]> SelectedTab_Set_TestData()
@@ -3951,17 +3951,17 @@ public void TabControl_GetToolTipText_Invoke_ReturnsExpectd()
         }
 
         [WinFormsFact]
-        public void TabControl_GetToolTipText_NullItem_ThrowsNullReferenceException()
+        public void TabControl_GetToolTipText_NullItem_ThrowsArgumentNullException()
         {
             using var control = new SubTabControl();
-            Assert.Throws<NullReferenceException>(() => control.GetToolTipText(null));
+            Assert.Throws<ArgumentNullException>("item", () => control.GetToolTipText(null));
         }
 
         [WinFormsFact]
-        public void TabControl_GetToolTipText_ItemNotTabPage_ThrowsInvalidCastException()
+        public void TabControl_GetToolTipText_ItemNotTabPage_ThrowsArgumentException()
         {
             using var control = new SubTabControl();
-            Assert.Throws<InvalidCastException>(() => control.GetToolTipText(new object()));
+            Assert.Throws<ArgumentException>("item", () => control.GetToolTipText(new object()));
         }
 
         public static IEnumerable<object[]> IsInputKey_TestData()