Fix the NPE on selection string on unlearnWord

Bug: 20150386
Change-Id: I2ed69aba7b1fdaba1e2ac07b8218a265dfe23e09

java/src/com/android/inputmethod/latin/RichInputConnection.java

@@ -48,6 +48,7 @@ import com.android.inputmethod.latin.utils.SpannableStringUtils;
 import com.android.inputmethod.latin.utils.TextRange;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 
 /**
  * Enrichment class for InputConnection to simplify interaction and add functionality.
@@ -288,6 +289,7 @@ public final class RichInputConnection implements PrivateCommandPerformer {
         }
     }
 
+    @Nullable
     public CharSequence getSelectedText(final int flags) {
         return isConnected() ?  mIC.getSelectedText(flags) : null;
     }

java/src/com/android/inputmethod/latin/inputlogic/InputLogic.java

@@ -1089,8 +1089,11 @@ public final class InputLogic {
                 // If there is a selection, remove it.
                 // We also need to unlearn the selected text.
                 final CharSequence selection = mConnection.getSelectedText(0 /* 0 for no styles */);
-                unlearnWord(selection.toString(), inputTransaction.mSettingsValues,
-                        Constants.EVENT_BACKSPACE);
+                if (!TextUtils.isEmpty(selection)) {
+                    unlearnWord(selection.toString(), inputTransaction.mSettingsValues,
+                            Constants.EVENT_BACKSPACE);
+                    hasUnlearnedWordBeingDeleted = true;
+                }
                 final int numCharsDeleted = mConnection.getExpectedSelectionEnd()
                         - mConnection.getExpectedSelectionStart();
                 mConnection.setSelection(mConnection.getExpectedSelectionEnd(),