CATROID-1479 Test: testFunctionDeletion fails

The test was failing because the back button in the formula editor was
not set active again after exiting the function menu. I fixed this
adding a call to set it active if necessary. Now the behaviour should be
as intended and the test passes.

catroid/src/main/java/org/catrobat/catroid/formulaeditor/FormulaEditorEditText.java

@@ -399,6 +399,9 @@ private String updateTextAndCursorFromInternFormula() {
 		}
 
 		highlightSelection();
+		if (formulaEditorFragment != null) {
+			formulaEditorFragment.updateButtonsOnKeyboard();
+		}
 
 		return newExternFormulaString;
 	}
@@ -426,6 +429,9 @@ public FormulaEditorHistory getHistory() {
 	}
 
 	public boolean isThereSomethingToDelete() {
+		if (internFormula == null) {
+			return false;
+		}
 		return internFormula.isThereSomethingToDelete();
 	}
 

catroid/src/main/java/org/catrobat/catroid/ui/fragment/FormulaEditorFragment.java

@@ -1076,7 +1076,10 @@ public void onHiddenChanged(boolean hidden) {
 
 	public void updateButtonsOnKeyboardAndInvalidateOptionsMenu() {
 		getActivity().invalidateOptionsMenu();
+		updateButtonsOnKeyboard();
+	}
 
+	public void updateButtonsOnKeyboard() {
 		ImageButton backspaceOnKeyboard = getActivity().findViewById(R.id.formula_editor_keyboard_delete);
 		if (!formulaEditorEditText.isThereSomethingToDelete()) {
 			backspaceOnKeyboard.setAlpha(255 / 3);