NullReferenceException when Firing an Event
What is a NullReferenceException, and how do I fix it?
Why doesn't a \foreach variable work in \draw in an \axis? I'm finding some weird behavior with TikZ's \foreach within an \begin{axis} environment.  This works:  \foreach \x in {-2,-1,...,2} {   \addplot[thick, domain=0.05+(\x*2-1)*pi/2:+(\x*2+1)*pi/2-0.05] (x, {tan(deg(x))});  }   but this doesn't:  \foreach \x in {-2,-1,...,2} {   \draw (\x,-5) -- (\x,5); }   It gives the error  ! Undefined control sequence. &lt;argument&gt; \x                ,-5 l.771     \end{axis}   Here's the really weird part: it only fails when the axis has defined bounds. For example, here's a MWE that works (but doesn't draw anything):  \documentclass{standalone} \usepackage{pgfplots} \begin{document} \begin{tikzpicture}   \begin{axis} % [ ymin=-3, ymax=3, xmin=-5, xmax=5]     \foreach \q in {-2,-1,...,2} {       \draw (axis cs: \q,-5) -- (axis cs: \q,5);     }   \end{axis} \end{tikzpicture} \end{document}   but removing the comment causes it to fail.  Why is this and how can I prevent it?  (My end goal is to draw five segments of the tangent function and their verical symptotes.) If $-1&lt;x&lt;1$ show that $\lim\limits_{n \to \infty}{n\,x^n} = 0$. I don't have idea. I only prove that $n\,x^n$ is decreasing. How to discard unsaved changes to a view when editing it leads to a WSOD? How to setup Windows from an external hard drive? Is it possible to install Windows from an external hard drive?  I have Windows XP installed on my laptop, and I want to remove it and install Windows 7.  My Windows 7 installation is in an .iso format on an external USB hard drive.  Is it possible to install it or will I need to put in a DVD before I start installing? What does the "Alternative Character Sheet" mean when it asks you for the "Maximum Hit Dice" and the "Remaining Hit Dice"? I am trying to fill in the "Alternative Character Sheet" (one of the ). It asks me for my "Maximum Hit Dice" and the "Remaining Hit Dice". What do these terms mean? Are Playstation 4 games locked to a certain region? This question has been asked before, but it was for PS3 and Xbox 360 and was asked August 2010. This time around my question goes specifically to PS4, since I'm moving from Mexico to Europe and I was wondering if the European games will run in my console or if I'll have to buy a new one. I never thought of this before (which is why I never paid attention if it was mentioned in reports or specs of the console) and the research I did wasn't too helpful. How to install latest HPLIP on my Ubuntu to support my HP printer and/or scanner? Let's assume that one has some modern HP printer and/or scanner which is not supported by HPLIP package from official repository. Currently Ubuntu versions have :  trusty (14.04LTS): 3.14.3-0ubuntu3.4 xenial (16.04LTS): 3.16.3+repack0-1 bionic (18.04LTS): 3.17.10+repack0-5 focal (20.04LTS): 3.20.3+dfsg0-2 groovy (20.10): 3.20.5+dfsg0-3build1 hirsute (21.04): 3.21.2+dfsg1-2   What can one do if printer is supported in newer version of HPLIP (checked this in )? When I install a simple program it often uses make &amp;&amp; make install and doesn't often even have an uninstall target.  If I wish to upgrade a program, is it standard protocol to assume it just rewrites seamlessly over the old program?  How do I keep track of these programs; do most people just 'fire and forget' and if no uninstall target is given do I have to manually delete everything? Where can I most efficiently buy all items for the "Shut up and take my money" achievement? Is particle superposition reflected in the particle's gravitational footprint? How to prove floor identities? Convert XML/HTML Entities into Unicode String in Python What should my Objective-C singleton look like? Regular Expressions: Is there an AND operator? Obviously, you can use the | (pipe?) to represent OR, but is there a way to represent AND as well?  Specifically, I'd like to match paragraphs of text that contain ALL of a certain phrase, but in no particular order. Is there a way to take a screenshot using Java and save it to some sort of image? I need to prove that:      if a linear operator $\phi : V \rightarrow V$ on a vector space is nilpotent, then its only eigenvalue is $0$.   I know how to prove that this for a nilpotent matrix, but I'm not sure in the case of an operator. How would I be able to relate $\phi$ to a matrix?  Note: A nilpotent operator $\phi$ has been defined as an operator that satisfies $\phi^{n} = 0$ for some $n \geq 1$. Every finite group is isomorphic to some Galois group for some finite normal extension of some field. How do I make a user's browser blink/flash/highlight in the task bar using JavaScript? For example, if I make an AJAX request every 10 seconds to see if the user has any new messages on the server, I want the user to know it right away, even if he is using another application at the time.  Edit: These users do want to be distracted when a new message arrives. How to unplug a network cable that's stuck? How can I set my system to suspend when inactive to a longer amount of time I run Mythtv on some Ubuntu 11.10 based machines.  In the power settings, I can choose from a list of time periods from 5 minutes up to 1 hour for the the machine to suspend when inactive.  I would like to set the suspend to occur at a longer interval, like 2 hours or 4.  Previously with Gnome, I would think that I could go into the gconf editor and find the key there, I am not sure with Unity where to override a setting like this.  Any suggestions?