The analyzer has detected an issue that has to do with checking a variable for 'null' after it has been used (in a method call, attribute access, and so on). This diagnostic operates within one logical expression.
Consider the following example:
if (rootDoc.Text.Trim() == documentName.Trim() && rootDoc != null)
In this code, attribute 'Text' is accessed first (moreover, method 'Trim' is called for this attribute), and only then the 'rootDoc' reference is checked for 'null'. If it proves to be equal to 'null', a 'NullReferenceException' will be raised. This bug can be fixed by having the referenced checked first and only then accessing the object's attribute:
if (rootDoc != null && rootDoc.Text.Trim() == documentName.Trim())
This is the simplest way to fix the error. However, you should carefully examine the code to figure out how to fix it best in every particular case.