Trying to fix build failures

ui/frame/frame.android.ts

@@ -512,6 +512,13 @@ class AndroidFrame extends Observable implements definition.AndroidFrame {
 }
 
 function findPageForFragment(fragment: android.app.Fragment, frame: Frame) {
+    if (!fragment) {
+        if (trace.enabled) {
+            trace.write(`Can't find page for a null fragment.`, trace.categories.NativeLifecycle);
+        }
+        return;
+    }
+
     var fragmentTag = fragment.getTag();
     var page: pages.Page;
     var entry: definition.BackstackEntry;
@@ -698,15 +705,12 @@ class FragmentClass extends android.app.Fragment {
         super.onDestroy();
 
         this.entry.fragmentTag = undefined;
-        this.entry = undefined;
-        this.frame = undefined;
-        this.clearHistory = undefined;
         transitionModule._clearBackwardTransitions(this);
         transitionModule._clearForwardTransitions(this);
     }
 
     public toString(): string {
-        return `${this.getTag()}<${(this.entry && this.entry.resolvedPage) ? this.entry.resolvedPage : ""}>`;
+        return `${this ? this.getTag() : ""}<${(this.entry && this.entry.resolvedPage) ? this.entry.resolvedPage : ""}>`;
     }
 }
 

ui/transition/transition.android.ts

@@ -35,6 +35,9 @@ export module AndroidTransitionType {
 }
 
 export function _clearBackwardTransitions(fragment: any): void {
+    if (!fragment) {
+        return;
+    }
     var expandedFragment = <ExpandedFragment>fragment;
     if (expandedFragment.enterPopExitTransition) {
         if (trace.enabled) {
@@ -62,6 +65,9 @@ export function _clearBackwardTransitions(fragment: any): void {
 }
 
 export function _clearForwardTransitions(fragment: any): void {
+    if (!fragment) {
+        return;
+    }
     var expandedFragment = <ExpandedFragment>fragment;
     if (expandedFragment.exitPopEnterTransition) {
         if (trace.enabled) {