I have a DrawerLayout, but when I add any view to the  content_View.xml I get this error:  java.lang.NullPointerException: Attempt to invoke interface method  'void android.support.v7.widget.DecorContentParent. setWindowCallback(android.view.Window$Callback)' on a null object reference  Can anyone explain why is this error happening and how to fix it?
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
How should we handle downvote retaliation? I have a situation where I'm trying to figure out what the right thing to do is on SO.    I see a question.  I answer the question.  I see there are a couple other answers.  One answers the question well, though with less explanation than my answer.  The other answer seems both wrong and irrelevant to the question (doesn't answer the question at all).  I comment to that effect.  The "wrong" part is edited out, but the author of the answer doesn't respond to how their answer actually answers the question.  In fact, they seem offended that I don't "get" it.  After giving them several chances to explain how their answer is relevant to the question and seeing their attitude, I decide to downvote their answer as I don't see how it addresses the question at all and the author won't explain.  They retaliate and downvote my answer and admit in their comments that their downvote of my answer has nothing to do with the content of my answer and only because they don't like me disputing their answer.  Did I do the right thing by downvoting their answer?  Do I just have to live with their downvote retaliation?  Anything I should have done differently?  I also have no way of knowing this for sure, but the next question I answered was upvoted, then downvoted so perhaps this same person is stalking my answers and downvoting other answers of mine too with no comment.  As long as this doesn't continue, I'm not losing sleep over a couple downvotes, but I do wonder if I should have done anything differently.  Followup Later After I Posted This Question: After thinking about it for a little while, the person who disagreed with me decided to delete their post.  So, eventually the message did get through to them.  I don't know if they rescinded their downvote or not (or if they even could). Data from multiple lists using single REST API call What are the applications of the ## preprocessor operator and gotchas to consider? As mentioned in many of my previous questions, I'm working through K&amp;R, and am currently into the preprocessor. One of the more interesting things &mdash; something I never knew before from any of my prior attempts to learn C &mdash; is the ## preprocessor operator. According to K&amp;R:     The preprocessor operator ##   provides a way to concatenate actual   arguments during macro expansion. If a   parameter in the replacement text is   adjacent to a ##, the parameter is   replaced by the actual argument, the   ## and surrounding white space are   removed, and the result is re-scanned.   For example, the macro paste   concatenates its two arguments:      #define  paste(front, back)  front ## back      so paste(name, 1) creates the token   name1.   How and why would someone use this in the real world? What are practical examples of its use, and are there gotchas to consider? Only squares instead of letters and numbers are displayed in my calculator and other apps in Ubuntu 18.04 I don't know what I did wrong in my Ubuntu 18.04 OS. For some reason some of my apps look like this:    This also happens to other gnome apps like pdf-reviewer and Logs. Window title, menu bar etc. are all blank, display only squares.  *Problem solved: I downloaded Google Fonts, saved them in a new folder, they can be applied system-wide, but after I did some font settings using Google Fonts it didn't work. I changed the font of Interface with Tweaks-Tool, that was what causing the problem. How do we know the Taylor expansion for $e^x$ works for all $x$? Or that it's analytic? How does Mage Armor interact with Unarmored Defense? collapse vs minimize In a Discord call on the desktop app and my voice cannot be heard by others. When I use Discord  through browser I can be heard. Any help appreciated. How to make `sudo` preserve $PATH? I have a program that is installed in a custom directory under /opt. To make it easier to run it, I edited my bashrc to add said directory to my path: export PATH=$PATH:/opt/godi/bin:/opt/godi/sbin  This works fine if I want to run the program without sudo. However, if I try to run it with sudo it fails with a &quot;command not found&quot; error. $ sudo godi_console sudo: godi_console: command not found  Inspecting the PATH variable after using sudo reveals that its not including the same PATH I have as a normal user: $ sudo sh # echo $PATH                  /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin  Why is the PATH not the same? Am I doing something wrong? I'm on Debian Jessie, if it makes a difference. One thing I tried was to invoke /opt/godi/sbin/godi_console directly, passing the absolute path to the executable. Unfortunatelly, that didn't help in this particular case because godi_console itself depends on the PATH being correctly set. RMSE vs Standard deviation in population My cursor is moving by every selection by large increments!  Normally when I use Blender, I am able to select and move objects to any coordinate. Now, all of the sudden, when I select an object or mesh the selection moves by an inch or so in each direction.  How do I reset it to normal? Why are some applications so hard to find in the Ubuntu Software Centre? Some applications don't pop up in the search even when you type in their name. Take for instance Sushi. When you type in "sushi" into the search box, it fails to throw up the application. Only when you type in "gnome-sushi" does it appear. This is despite the fact that the application has "sushi" by itself in the application description. There are other applications which don't spring to mind right now, but I've discovered before. Adding popup message box before tool execution with ArcPy and ArcMap I want to add a box message to pop up before execution of a button tool. When I click a button which will do geoprocessing, I want a box to appear on view before processing with message that I am about to do xxx processing. How do I do it? There does not exist any integer $m$ such that $3n^2+3n+7=m^3$ I have this really hard problem that I am working on and I just don't seem to get it. The question is: let $n$ be a positive integer; prove that there does not exist any integer $m$ such that $3n^2+3n+7=m^3$.  Please explain in detail otherwise I might not understand. I am also open for any methods that might help. Thank you. Please help. Let $f:X\to Y$ be a function and $E\subseteq X$ and $F\subseteq X$.   Show that in general $f(E − F)\nsubseteq f(E) − f(F)$.   I have no idea about how to prove this; and could anyone please explain the basic theory of functions(by relating to this question. Thank you. Symbol for “such that” (not in set) Prove $e^{x+y}=e^{x}e^{y}$ by using Exponential Series I presented the following problem to some of my students recently (from Senior Mathematical Challenge- edited by Gardiner)  In the Fibonacci sequence $1, 1, 2, 3, 5, 8, 13, 21, 34, 55,\ldots$ each term after the first two is the sum of the two previous terms. What is the sum to infinity of the series: $$\frac{1}{2} + \frac{1}{4}+ \frac{2}{8} + \frac{3}{16} + \frac{5}{32} +\frac{8}{64} + \frac{13}{128} +\frac{21}{256} +\frac{34}{512}+ \frac{55}{1024} + \cdots$$  Now, I solved this using an infinite geometric matrix series (incorporating the matrix version of the relation $a_n= \frac{a_{n-1}}{2}+ \frac{a_{n-2}}{4}$), and my students, after much hinting on my part, googled the necessary string to stumble across Binet's formula (which allows one to split the series into two simple, if rather messy, geometrics). Both of these are good methods, but neither really seems plausible for a challenge set for 15-18 year olds under exam conditions. So how is one supposed to do it? How can I raise a soldiers hacking ability? My main specialist seems to have a hard time hacking stuff. How can I make him better at it? What determines the number of comments that are displayed? I'm curious how it is decided how many comments to display before the "Show more comments" link. Sometimes I see a dozen comments on a question and no "Show more comments" link, in other occasions no comments are visible at all, only the link.