Android Studio cannot resolve symbol but code executes correctly

I have faced this issue when IntelliJ IDEA got closed abruptly due to OS crash.

You can do "File" -> "Invalidate Caches...", and select "Invalidate and Restart" option to fix this.

EDIT: For most folks, Senthil's answer will be more appropriate. I am leaving this one as the accepted answer because it solved my specific problem.

I found the issue - my SDK.jar was not generated correctly. It included .java files instead of .class files. This explains why the IDE was not able to find the SDK class. The package structure was still correct in the Jar, which is why the package name itself is not a red color. The code worked correctly, because the compiler knew to compile the .java files.

To solve the issue, I modified my build.gradle of my SDK project to include .class files, instead of .java files, when creating the Jar. Including this new Jar instead of the old Jar fixed the IDE issue.