Adding a primary key to table in a fault-tolerant manner

AddPK.sql

IF 0 < (
	SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES 
	WHERE TABLE_TYPE = 'BASE TABLE'
	AND TABLE_SCHEMA = 'dbo'
	AND TABLE_NAME = 'T_SYS_Language_Forms'
)
BEGIN
	-- Check for NULL values in the primary-key column
	IF 0 = (SELECT COUNT(*) FROM T_SYS_Language_Forms WHERE LANG_UID IS NULL)
	BEGIN
		ALTER TABLE T_SYS_Language_Forms ALTER COLUMN LANG_UID uniqueidentifier NOT NULL 
		
		-- Drop PK if exists 
		-- ALTER TABLE T_SYS_Language_Forms DROP CONSTRAINT pk_constraint_name 
		--DECLARE @pkDropCommand nvarchar(1000) 
		--SET @pkDropCommand = N'ALTER TABLE T_SYS_Language_Forms DROP CONSTRAINT ' + QUOTENAME((SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
		--WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
		--AND TABLE_SCHEMA = 'dbo' 
		--AND TABLE_NAME = 'T_SYS_Language_Forms' 
		----AND CONSTRAINT_NAME = 'PK_T_SYS_Language_Forms' 
		--))
		---- PRINT @pkDropCommand 
		--EXECUTE(@pkDropCommand) 
		
		
		-- Check if they keys are unique		
		IF 1 >= (SELECT TOP 1 COUNT(*) AS cnt FROM T_SYS_Language_Forms GROUP BY LANG_UID ORDER BY cnt DESC)
		BEGIN
			
			-- If no Primary key for this table
			IF 0 =  
			(
				SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
				WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
				AND TABLE_SCHEMA = 'dbo' 
				AND TABLE_NAME = 'T_SYS_Language_Forms' 
				-- AND CONSTRAINT_NAME = 'PK_T_SYS_Language_Forms' 
			)
				ALTER TABLE T_SYS_Language_Forms ADD CONSTRAINT PK_T_SYS_Language_Forms PRIMARY KEY CLUSTERED (LANG_UID ASC)
			;
			
		END -- End uniqueness check
		ELSE
			PRINT 'FSCK, this column has duplicate keys, and can thus not be changed to primary key...' 
	END -- End NULL check
	ELSE
		PRINT 'FSCK, need to figure out how to update NULL value(s)...' 
END