Avoid null context in fragments.

[pnemonic78]

Fatal Exception: java.lang.IllegalStateException: Fragment TransactionFormFragment{249d286} (ad28f118-0970-4ee9-a7ee-24e1d56829ae) not attached to a context.
       at androidx.fragment.app.Fragment.requireContext(Fragment.java:967)
       at org.gnucash.android.ui.transaction.TransactionFormFragment.onOptionsItemSelected(TransactionFormFragment.java:941)
       at org.gnucash.android.app.MenuFragment$onCreate$1.onMenuItemSelected(MenuFragment.kt:23)
       at androidx.core.view.MenuHostHelper.onMenuItemSelected(MenuHostHelper.java:107)
       at androidx.activity.ComponentActivity.onMenuItemSelected(ComponentActivity.java:532)
       at androidx.fragment.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:265)
       at androidx.appcompat.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:269)
       at androidx.appcompat.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:109)
       at androidx.appcompat.app.ToolbarActionBar$2.onMenuItemClick(ToolbarActionBar.java:66)
       at androidx.appcompat.widget.Toolbar$1.onMenuItemClick(Toolbar.java:224)
       at androidx.appcompat.widget.ActionMenuView$MenuBuilderCallback.onMenuItemSelected(ActionMenuView.java:769)
       at androidx.appcompat.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:833)
       at androidx.appcompat.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:157)
       at androidx.appcompat.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:984)
       at androidx.appcompat.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:974)
       at androidx.appcompat.widget.ActionMenuView.invokeItem(ActionMenuView.java:620)
       at androidx.appcompat.view.menu.ActionMenuItemView.onClick(ActionMenuItemView.java:155)
       at android.view.View.performClick(View.java:7535)
       at android.view.View.performClickInternal(View.java:7512)
       at android.view.View.-$$Nest$mperformClickInternal()
       at android.view.View$PerformClick.run(View.java:29496)
       at android.os.Handler.handleCallback(Handler.java:984)
       at android.os.Handler.dispatchMessage(Handler.java:104)
       at android.os.Looper.loopOnce(Looper.java:238)
       at android.os.Looper.loop(Looper.java:357)
       at android.app.ActivityThread.main(ActivityThread.java:8194)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:957)